build: update README to indicate that source has moved (#1969)

Towards https://github.com/googleapis/google-cloud-python/issues/14908

README.rst

diff --git a/README.rst b/README.rst
index 45da804..380c2f0 100644
--- a/README.rst
+++ b/README.rst
@@ -1,3 +1,8 @@
+:**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/google-auth
+
+
 Google Auth Python Library
 ==========================