Add S3 gotcha - incomplete multi-part upload costs

Each uploaded part of a multi-part upload begins accruing
storage charges immediately, even if the upload as a whole
fails. This can really add up for certain usage patterns,
and can fly under the radar easily unless you know to
look for it.

Include quick summary of this gotcha and some links (Amazon
and third-party) for more detailed information.

README.md

@@ -699,6 +699,7 @@ S3
 -	🔸After uploading, any change that you make to the object causes a full rewrite of the object, so avoid appending-like behavior with regular files.
 -	🔸Eventual data consistency, as discussed above, can be surprising sometimes. If S3 suffers from internal replication issues, an object may be visible from a subset of the machines, depending on which S3 endpoint they hit. Those usually resolve within seconds; however, we’ve seen isolated cases when the issue lingered for 20-30 hours.
 -	🔸**MD5s and multi-part uploads:** In S3, the [ETag header in S3](http://docs.aws.amazon.com/AmazonS3/latest/API/RESTCommonResponseHeaders.html) is a hash on the object. And in many cases, it is the MD5 hash. However, this [is not the case in general](http://stackoverflow.com/questions/12186993/what-is-the-algorithm-to-compute-the-amazon-s3-etag-for-a-file-larger-than-5gb) when you use multi-part uploads. One workaround is to compute MD5s yourself and put them in a custom header (such as is done by [s4cmd](https://github.com/bloomreach/s4cmd)).
+-	🔸**Incomplete multi-part upload costs:** Incomplete multi-part uploads accrue [storage charges](http://docs.aws.amazon.com/AmazonS3/latest/dev/mpuoverview.html#mpuploadpricing) even if the upload fails and no S3 object is created. [Amazon](http://docs.aws.amazon.com/AmazonS3/latest/dev/mpuoverview.html#mpu-abort-incomplete-mpu-lifecycle-config) ([and](http://www.deplication.net/2016/06/aws-tip-save-s3-costs-with-abort.html) [others](https://www.sumologic.com/aws/s3/s3-cost-optimization/)) recommend using a lifecycle policy to clean up incomplete uploads and save on storage costs.
 -	🔸**US Standard region:** Previously, the us-east-1 region (also known as the US Standard region) was replicated across coasts, which led to greater variability of latency. Effective Jun 19, 2015 this is [no longer the case](https://forums.aws.amazon.com/ann.jspa?annID=3112). All Amazon S3 regions now support read-after-write consistency. Amazon S3 also renamed the US Standard region to the US East (N. Virginia) region to be consistent with AWS regional naming conventions.
 -	❗When configuring ACLs on who can access the bucket and contents, a predefined group exists called **[Authenticated Users](http://docs.aws.amazon.com/AmazonS3/latest/dev/acl-overview.html)**. This group is often used, incorrectly, to restrict S3 resource access to authenticated users of the owning account. If granted, the AuthenticatedUsers group will allow S3 resource access to **all authenticated users, across all AWS accounts**. A typical use case of this ACL is used in conjunction with the [requester pays](http://docs.aws.amazon.com/AmazonS3/latest/dev/RequesterPaysBuckets.html) functionality of S3.
 - 🔸**S3 authentication versions and regions:** In newer regions, S3 [only supports the latest authentication](https://docs.aws.amazon.com/AmazonS3/latest/dev/UsingAWSSDK.html#specify-signature-version). If an S3 file operation using CLI or SDK doesn't work in one region, but works correctly in another region, make sure you are using the latest [authentication signature](https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-authenticating-requests.html).