Improve README with a minimal S3 policy

[solracsf]

The Minimal Policy would recommend the minimal rights needed for the application to run. Ex.:

GetObject
PutObject
DeleteObject

or a JSON:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:PutObject",
        "s3:GetObject",
        "s3:DeleteObject"
      ],
      "Resource": ["arn:aws:s3:::bucketName/*"]
    }
  ]
}