Add documentation around oauth2client deprecation (#165)

diff --git a/docs/index.rst b/docs/index.rst
index c0cbdbf..b6dc92c 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -22,6 +22,8 @@
   :mod:`urllib3 <google.auth.transport.urllib3>`, and
   :mod:`gRPC <google.auth.transport.grpc>`.
 
+.. note:: ``oauth2client`` was recently deprecated in favor of this library. For more details on the deprecation, see :doc:`oauth2client-deprecation`.
+
 Installing
 ----------