Update README.md (#7491)

Adding explicit licensing instructions

Author: brmur

README.md

@@ -100,3 +100,8 @@ Copyright © Amazon Web Services, Inc. or its affiliates. All rights reserved.
 Except where otherwise noted, all examples in this collection are licensed under the [Apache
 license, version 2.0](https://www.apache.org/licenses/LICENSE-2.0) (the "License"). The full
 license text is provided in the `LICENSE` file accompanying this repository.
+
+Please include the following licensing text as a comment at top of all possible files:
+
+*Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+SPDX-License-Identifier: Apache-2.0*