Update README/Docs to include a note writing to S3 when you have multiple accounts

[aabdullah-bos]

In a world, where you need to run papermill across different accounts. You can set the AWS_PROFILE environment variable. So that papermill uses the appropriate credentials to store the results of a notebook execution. I posted this in slack, and I am creating this issue so that it can be associated with a pull request to update the docs.