More restrictive permissions? about kube2iam

[jtblin]

Updated README as well.

[jtblin]

@mgoodness I'll update the readme, but basically just replace root in the Trust Relationship by the name of the role for the node that kubernetes worker is deployed on e.g.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": "ec2.amazonaws.com"
      },
      "Action": "sts:AssumeRole"
    },
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": [
          "arn:aws:iam::123456789012:role/platform-dev-kubernetes-common-NodeIAMRole-7WEHDM7P3F5C"
        ]
      },
      "Action": "sts:AssumeRole"
    }
  ]
}