Add EC2 gotcha about burst performance

README.md

@@ -726,6 +726,7 @@ EC2
 -	🔸Periodically you may find that your server or load balancer is receiving traffic for (presumably) a previous EC2 server that was running at the same IP address that you are handed out now (this may not matter, or it can be fixed by migrating to another new instance).
 -	❗If the EC2 API itself is a critical dependency of your infrastructure (e.g. for automated server replacement, custom scaling algorithms, etc.) and you are running at a large scale or making many EC2 API calls, make sure that you understand when they might fail (calls to it are [rate limited](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/query-api-troubleshooting.html#api-request-rate) and the limits are not published and subject to change) and code and test against that possibility.
 -	❗Many newer EC2 instance types are EBS-only. Make sure to factor in EBS performance and costs when planning to use them.
+-	🔸Instances come in two types: Fixed Performance Instances (e.g. M3, C3, and R3) and Burstable Performance Instances (e.g. T2). Each T2 instance receives CPU Credits continuously, the rate of which depends on the instance size. T2 instances accrue CPU Credits when they are idle, and use CPU credits when they are active. However, once it runs out of credits, you'll notice a severe degrade in performance. CPU-bound tasks (such as scientific computation) are therefore better suited for Fixed Performance Instances.
 
 AMIs
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