chore: add repo deprecation message

Author: miguelvelezsa

README.md

@@ -1,3 +1,6 @@
+**_THIS REPOSITORY IS DEPRECATED. ALL OF ITS CONTENT AND HISTORY HAS BEEN MOVED TO [GOOGLE-CLOUD-NODE-CORE](https://github.com/googleapis/google-cloud-node-core/tree/main/packages/google-auth-library-nodejs)_**
+
+
 [//]: # "This README.md file is auto-generated, all changes to this file will be lost."
 [//]: # "To regenerate it, use `python -m synthtool`."
 <img src="https://avatars2.githubusercontent.com/u/2810941?v=3&s=96" alt="Google Cloud Platform logo" title="Google Cloud Platform" align="right" height="96" width="96"/>