Document minimal S3 permissions

[axelfontaine]

The minimal IAM policy should be something like

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "github-actions-cache-server",
            "Effect": "Allow",
            "Action": [
                "s3:PutObject",
                "s3:GetObject",
                "s3:ListBucket",
                "s3:DeleteObject",
                "s3:GetBucketLocation"
            ],
            "Resource": [
                "arn:aws:s3:::*"
            ]
        }
    ]
}