Update README prior to archiving

Author: joshbean

README.md

@@ -1,3 +1,7 @@
+# NOTICE
+
+**This repository is archived and the content on this branch is out of date.** You can find up-to-date AWS technical documentation on the [AWS Documentation website](https://docs.aws.amazon.com/), where you can also submit feedback and suggestions for improvement.
+
 ## Amazon API Gateway Developer Guide
 
 The open source version of the Amazon API Gateway Developer Guide. To submit feedback or requests for changes, submit an issue or make changes and submit a pull request.