Add example

README.md

@@ -30,3 +30,7 @@ of processes to make the actual calls to the AWS API.
  * `new_acl`: The new acl set set on the matched objects. The Amazon canned ACLs
    are: `private`, `public-read`, `public-read-write`, `authenticated-read`,
    `aws-exec-read`, `bucket-owner-read`, `bucket-owner-full-control`
+
+#### Example
+
+    mass_s3 put-object-acl -m 2020-03-01 -o other-account-alias my-files 2020/ bucket-owner-full-control