Recommend new ADC command

Author: Bill Prin

docs/gcloud-auth.rst

@@ -12,10 +12,22 @@ 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. If your gcloud installation does not support the new command, please update it:
+
+    .. code-block:: bash
+
+        $ gcloud components update
+
 .. _Google Cloud SDK: http://cloud.google.com/sdk