link to oauth2client deprecation explanation. (googleapis#728)

Author: jay0lee

README.md

@@ -6,7 +6,7 @@ This is a client library for accessing resources protected by OAuth 2.0.
 
 **Note**: oauth2client is now deprecated. No more features will be added to the
 libraries and the core team is turning down support. We recommend you use
-[google-auth](https://google-auth.readthedocs.io) and [oauthlib](http://oauthlib.readthedocs.io/).
+[google-auth](https://google-auth.readthedocs.io) and [oauthlib](http://oauthlib.readthedocs.io/). For more details on the deprecation, see [oauth2client deprecation](https://google-auth.readthedocs.io/en/latest/oauth2client-deprecation.html).
 
 Installation
 ============