Document S3 retention levels/required permissions

[mdedetrich]

What is currently missing?

When using the S3 storage engine we should document what permissions are needed for S3 accounts that Guardian.

How could this be improved?

Since Guardian uses non typical parts of the S3 API which need extra permissions, i.e. ListParts) this needs to be documented since it can come off as a surprise.

Also retention levels for uploaded parts with multipart uploads also need to be documented, i.e. if the retention level of your multipart upload is lower than the configured timeslice you will get problems since the parts will disappear before they get completed.

Is this a feature you would work on yourself?

• I plan to open a pull request for this feature