Recommend new ADC command

Author: Bill Prin

docs/gcloud-auth.rst

@@ -12,10 +12,18 @@ Overview
 *   **If you're developing locally**,
     the easiest way to authenticate is using the `Google Cloud SDK`_:
 
+    .. code-block:: bash
+        
+        $ gcloud beta auth application-default login
+
+    Note that this command generates credentials for client libraries. To authenticate the CLI itself, use: 
+
     .. code-block:: bash
 
         $ gcloud auth login
 
+    Previously, `gcloud auth login` was used for both use cases.
+
 .. _Google Cloud SDK: http://cloud.google.com/sdk