Added Amazon IAM section in the README.md

When upgrading the plugin from 3.0.0.RELEASE to 5.0.0.RELEASE, I bumped into this permission issue where 5.0.0.RELEASE uses `AmazonS3Client.getBucketLocation()`,  which requires a new permission to be added compared to the 3.0.0.RELEASE.  I hope this note will save some time for others who are also seeking for upgrade as well.