Update README to add announcement about archiving

Author: joshbean

README.md

@@ -1,3 +1,9 @@
+# Announcement
+
+This repository will be archived and marked read-only next month (June 2023). For more information, read [the announcement on the AWS News Blog](https://aws.amazon.com/blogs/aws/retiring-the-aws-documentation-on-github/).
+
+You can find the corresponding content for this repo on [the AWS Documentation website](https://docs.aws.amazon.com/systems-manager/latest/userguide). If you'd like to continue contributing to the quality of AWS documentation, you can submit feedback and suggestions for improvement there.
+
 ## aws-systems-manager-user-guide
 
 The open source version of the AWS Systems Manager user guide. You can submit feedback & requests for changes by submitting issues in this repo or by making proposed changes & submitting a pull request.