Include instructions on how to setup a user on S3

These instructions where shameless copied from s3-upload package. I know that a glob on Actions is an evil thing, but these instructions are aimed at newbies.

Maybe a section could be included listing which permissions are needed for each operations (upload, sync, download etc.)

For simple upload operations, ListObjects, PutObjects and DeleteObjects where not enough. I simply couldn't make it work without a glob operation.