build: update README to indicate that source has moved (#1540)

Towards googleapis/google-cloud-python#10952.

Author: parthea

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-cloud-spanner
+
+
 Python Client for Cloud Spanner
 ===============================