Add typical IAM policy to README

[AntonUspishnyi]

I want to use this package in my CI-system, so I need to know what IAM policy create for everything works fine.

[AntonUspishnyi]

I think something like

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "ListBuckets",
            "Effect": "Allow",
            "Action": "s3:ListAllMyBuckets",
            "Resource": "arn:aws:s3:::*"
        },
        {
            "Sid": "OperateWithBucket",
            "Effect": "Allow",
            "Action": [
                "s3:PutObject",
                "s3:ListBucket",
                "s3:DeleteObject"
            ],
            "Resource": [
                "arn:aws:s3:::BUCKET_NAME",
                "arn:aws:s3:::BUCKET_NAME/*"
            ]
        },
        {
            "Sid": "CloudFrontInvalidations",
            "Effect": "Allow",
            "Action": "cloudfront:CreateInvalidation",
            "Resource": [
                "arn:aws:cloudfront::ACCOUNT_ID:distribution/DISTRIBUTION_ID"
            ]
        }
    ]
}

[apancutt]

The necessary policy permissions depend on the arguments used when calling the tool (additional permissions are required if invalidating CloudFront or setting object ACL).

I'll try to get the full scope documented in the README asap.

[apancutt]

The example provided by @AntonUspehov has been merged to master. Further examples will be required for advanced usage but closing for this issue now.