Add required AWS permissions for S3 bucket to docs

[fenech]

I am trying to use this resource with an S3 bucket and needed to add the required permissions to my AWS user. I got it to work by using the list of permissions mentioned on https://github.com/concourse/s3-resource:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "SemverReadWrite",
            "Effect": "Allow",
            "Action": [
                "s3:PutObject",
                "s3:GetObject",
                "s3:ListBucket",
                "s3:PutObjectAcl"
            ],
            "Resource": [
                "arn:aws:s3:::BUCKET_NAME/*",
                "arn:aws:s3:::BUCKET_NAME"
            ]
        }
    ]
}

Perhaps this information should be included in the README?