Instructions to set up AWS credentials in readme

[corinaminer]

People who haven't used AWS need to set up their local AWS configuration and credentials before they can use the S3 integration: https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-files.html

S3 section of readme should include this link as well as instructions for finding your aws_access_key_id and aws_secret_access_key. fyi @saparikh @sfraint