Add indication about Cellar principal policy

[gaelreyrol]

It is possible to apply specific rights using a policy but we don't give any information about which principal should be used.
In that case it is also possible to create a cellar add-on and give access (read-only for example) to another cellar add-on using its principal.

Cellar add-on principal should be used like this:

{
        "Id": "Policy1639060414161",
        "Version": "2012-10-17",
        "Statement": [
                {
                        "Sid": "Stmt1639060333099",
                        "Action": [
                                "s3:GetObject"
                        ],
                        "Effect": "Allow",
                        "Resource": "arn:aws:s3:::my-bucket/*",
                        "Principal": {
                                "AWS": "arn:aws:iam:::user/<cellar_id>"
                        }
                }
        ]
}

[gaelreyrol]

Customers should refer to this documentation to create policies: https://docs.ceph.com/en/nautilus/radosgw/bucketpolicy/

[cnivolle]

@Dekabry I Think this issue can be related to #471.