AWS S3: Add minimum policy rules for complainer

[andygrunwald]

I configured complainer with the usage of S3 (s3aws from #17). Everything is working fine. What i asked myself during this road:

• What Policies / Grants does complainer need to work with S3?
• What are the minimum grants?

I tested it and the only requirement that is needed is:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Stmt1465396834000",
            "Effect": "Allow",
            "Action": [
                "s3:PutObject"
            ],
            "Resource": [
                "arn:aws:s3:::YOUR-BUCKET-NAME/*"
            ]
        }
    ]
}

It would make sense to add this to the README / docs for future users. I didn`t apply a PR (yet), because #17 is not merged yet and this would create a conflict if i put this into the README as well.

[bobrik]

Looks like s3:PutObject should be enough. Feel free to make a PR to reflect that.

[andygrunwald]

See #22