Document required S3 bucket policy

[lindhe]

It's currently unclear what S3 bucket policy is required for the S3 support for etcd snapshots to work. Would be good if this was documented with an example!

[wszychta]

We are also facing this problem. This would be very usefull to add such documentation with minimum list of permissions.

[lindhe]

The policy I've applied looks like this:

{
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "s3:DeleteObject",
                "s3:GetObject",
                "s3:ListBucket",
                "s3:PutObject"
            ],
            "Resource": [
                "arn:aws:s3:::foo",
                "arn:aws:s3:::foo/*"
            ]
        }
    ]
}

I am experiencing some issues currently, but I think it's unrelated to the policy (but hard to tell for sure). So if we can get a review of this policy, perhaps that can be a basis for docs?