Update docs

docs/source/index.rst

@@ -88,6 +88,10 @@ Two modes of authentication are supported:
     - if ``token=None``, you will be given a "device code", which you must
       enter into a browser where you are logged in with your Google identity.
 
+    - if ``token='cloud'``, we assume we are running within google (compute
+      or container engine) and fetch the credentials automatically from the
+      metadata service.
+
     - you may supply a token generated by the
       gcloud_ utility; this is either a python dictionary, or the name of a file
       containing the JSON returned by logging in with the gcloud CLI tool. On