boto3 refactor - document credential handling changes

[jantman]

See this comment on #95 -

Look into how boto3 handles default regions. All of ALC lets the default region be None and passes this through to boto. With boto3, there's a change to the underlying environment variable use. I found this out by running awslimitchecker -vv -u -S ElastiCache after 32416b7, and got vastly different results from the latest release. It turns out that the new boto3 code is connecting to us-west-2. It appears that with boto2, if you set the AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY environment variables, it completely ignores ~/.aws/credentials. With boto3, however, if you have a [default] section in ~/.aws/credentials and then export the key env vars, it still uses the region from ~/.aws/credentials.