build: update README to indicate that source has moved (#2600)

Author: parthea

README.rst

@@ -1,3 +1,7 @@
+:**NOTE**: **This github repository is archived. The repository contents and history have moved to** `google-cloud-python`_.
+
+.. _google-cloud-python: https://github.com/googleapis/google-cloud-python/tree/main/packages/gapic-generator
+
 .. _codingstyle:
 
 API Client Generator for Python