Update README.md for aws-vault compatibility

[jonny-wg2]

It is required to also include the AWS_SESSION_TOKEN env variable when using aws-vault / Docker.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}