address Josh's comments

README.md

@@ -545,7 +545,7 @@ We cover security basics first, since configuring user accounts is something you
 -	At the beginning, IAM policy may be very simple, but for large systems, it will grow in complexity, and need to be managed with care.
 	-	🔹Make sure one person (perhaps with a backup) in your organization is formally assigned ownership of managing IAM policies, make sure every administrator works with that person to have changes reviewed. This goes a long way to avoiding accidental and serious misconfigurations.
 -	It is best to give each user or service the minimum privileges needed to perform their duties. This is the [principle of least privilege](https://en.wikipedia.org/wiki/Principle_of_least_privilege), one of the foundations of good security. Organize all IAM users and groups according to levels of access they need.
--	IAM has the permission hierarchy of:
+-	IAM has the permission [hierarchy of](http://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_evaluation-logic.html):
 	1. Explicit deny: The most restrictive policy wins.
 	2. Explicit allow: Access permissions to any resource has to be explicitly given.
 	3. Implicit deny: All permissions are implicitly denied by default.
@@ -687,7 +687,8 @@ S3
 -	If you are primarily using a VPC, consider setting up a [VPC Endpoint](http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/vpc-endpoints.html) for S3 in order to allow your VPC-hosted resources to easily access it without the need for extra network configuration or hops.
 -	**Cross-region replication:** S3 has [a feature](https://docs.aws.amazon.com/AmazonS3/latest/dev/crr.html) for replicating a bucket between one region and a another. Note that S3 is already highly replicated within one region, so usually this isn’t necessary for durability, but it could be useful for compliance (geographically distributed data storage), lower latency, or as a strategy to reduce region-to-region bandwidth costs by mirroring heavily used data in a second region.
 -	**IPv4 vs IPv6:** For a long time S3 only supported IPv4 at the default endpoint `https://BUCKET.s3.amazonaws.com`. However, [as of Aug 11, 2016](https://aws.amazon.com/blogs/aws/now-available-ipv6-support-for-amazon-s3/) it now supports both IPv4 & IPv6! To use both, you have to [enable dualstack](http://docs.aws.amazon.com/AmazonS3/latest/dev/dual-stack-endpoints.html) either in your preferred API client or by directly using this url scheme `https://BUCKET.s3.dualstack.REGION.amazonaws.com`.
--	**S3 Event Notifications:** S3 can be configured to send an [SNS notification](https://aws.amazon.com/blogs/aws/introducing-the-amazon-simple-notification-service/), [SQS message](http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/Welcome.html), or [AWS Lambda function](http://docs.aws.amazon.com/lambda/latest/dg/welcome.html) on bucket events. [doc](http://docs.aws.amazon.com/AmazonS3/latest/dev/NotificationHowTo.html)
+-	**S3 event notifications:** S3 can be configured to send an [SNS notification](https://aws.amazon.com/blogs/aws/introducing-the-amazon-simple-notification-service/), [SQS message](http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/Welcome.html), or [AWS Lambda function](http://docs.aws.amazon.com/lambda/latest/dg/welcome.html) on [bucket events](http://docs.aws.amazon.com/AmazonS3/latest/dev/NotificationHowTo.html).
+
 ### S3 Gotchas and Limitations
 
 -	🔸For many years, there was a notorious [**100-bucket limit**](https://docs.aws.amazon.com/general/latest/gr/aws_service_limits.html#limits_s3) per account, which could not be raised and caused many companies significant pain. As of 2015, you can [request increases](https://aws.amazon.com/about-aws/whats-new/2015/08/amazon-s3-introduces-new-usability-enhancements/). You can ask to increase the limit, but it will still be capped (generally below ~1000 per account).
@@ -1374,7 +1375,7 @@ Redshift
 -	❗ Never resize a live cluster. The resize operation takes hours depending on the dataset size. In rare cases, the operation may also get stuck and you'll end up having a non-functional cluster. The safer approach is to create a new cluster from a snapshot, resize the new cluster and shut down the old one.
 -	Redshift has reserved keywords which are not present in Postgres (see full list [here](https://docs.aws.amazon.com/redshift/latest/dg/r_pg_keywords.html)). Watch out for DELTA ([Delta Encodings](https://docs.aws.amazon.com/redshift/latest/dg/c_Delta_encoding.html)).
 -	Redshift does not support many Postgres functions, most notably several date/time-related and aggregation functions. See the [full list here](https://docs.aws.amazon.com/redshift/latest/dg/c_unsupported-postgresql-functions.html).
--	Compression on sortkey can result in significant performance impact. So if your Redshift queries involving sort key(s) are slow, you might want to consider removing compression on a sortkey. See [Data Compression](https://aws.amazon.com/blogs/big-data/optimizing-for-star-schemas-and-interleaved-sorting-on-amazon-redshift/).
+-	Compression on sortkey [can result in significant performance impact](https://aws.amazon.com/blogs/big-data/optimizing-for-star-schemas-and-interleaved-sorting-on-amazon-redshift/). So if your Redshift queries involving sort key(s) are slow, you might want to consider removing compression on a sortkey.
 -	🔹 [Choosing a sort key](http://docs.aws.amazon.com/redshift/latest/dg/t_Sorting_data.html) is very important since you can not change a table’s sort key after it is created. If you need to change the sort or distribution key of a table, you need to create a new table with the new key and move your data into it with a query like “insert into new_table select * from old_table”.
 -	❗🚪 When moving data with a query that looks like “insert into x select from y”, you need to have twice as much disk space available as table “y” takes up on the cluster’s disks. Redshift first copies the data to disk and then to the new table. [Here](https://www.periscopedata.com/blog/changing-dist-and-sort-keys-in-redshift.html) is a good article on how to this for big tables.
 
@@ -1395,8 +1396,7 @@ EMR
 -	EMR relies on many versions of Hadoop and other supporting software. Be sure to check [which versions are in use](https://docs.aws.amazon.com/ElasticMapReduce/latest/ReleaseGuide/emr-release-components.html).
 -	⏱Off-the-shelf EMR and Hadoop can have significant overhead when compared with efficient processing on a single machine. If your data is small and performance matters, you may wish to consider alternatives, as [this post](http://aadrake.com/command-line-tools-can-be-235x-faster-than-your-hadoop-cluster.html) illustrates.
 -	Python programmers may want to take a look at Yelp’s [mrjob](https://github.com/Yelp/mrjob).
--	Larger instances don't neccessarily cost more in spot market. Therefore, you should look at different options and determine which instances you should bid for your jobs. See [Bid Advisor](https://aws.amazon.com/ec2/spot/bid-advisor/).
--	Since EMR jobs are billed at hour granularity, it is usually beneficial to increase the number of instances and/or the type of instances to keep the time of running your EMR jobs under one hour.
+-	:money_with_wings: Hourly pricing roundoff: Since EMR jobs are billed at one-hour granularity, it can be beneficial to tune the number and/or type of instances so that jobs do not too quickly (if it runs for only a few minutes, you’ll still be billed for the full hour) or run just over an hour (where you’ll be billed for 2 hours). 
 -	It takes time to tune performance of EMR jobs, which is why third-party services such as [Qubole’s data service](https://www.qubole.com/mapreduce-as-a-service/) are gaining popularity as ways to improve performance or reduce costs.
 
 ### EMR Gotchas and Limitations
@@ -1589,6 +1589,7 @@ Billing and Cost Management
 ### EC2 Cost Management
 
 -	With EC2, there is a trade-off between engineering effort (more analysis, more tools, more complex architectures) and spend rate on AWS. If your EC2 costs are small, many of the efforts here are not worth the engineering time required to make them work. But once you know your costs will be growing in excess of an engineer’s salary, serious investment is often worthwhile.
+-	Larger instances don't neccessarily cost more in spot market. Therefore, you should look at different options and determine which instances you should bid for your jobs. See [Bid Advisor](https://aws.amazon.com/ec2/spot/bid-advisor/).
 -	🔹**Spot instances:**
 	-	EC2 [Spot instances](https://aws.amazon.com/ec2/spot/) are a way to get EC2 resources at significant discount — often many times cheaper than standard on-demand prices — if you’re willing to accept the possibility that they be terminated with little to no warning.
 	-	Use Spot instances for potentially very significant discounts whenever you can use resources that may be restarted and don’t maintain long-term state.