service account + docs link

Add reference to service account and a link to more documentation.

samples/call_compute_service.py

@@ -1,6 +1,8 @@
 # To be used to test GoogleCredentials.get_application_default()
 # from local machine and GCE.
-# The GCE virtual machine needs to have the Compute API enabled.
+# The GCE virtual machine needs to have both service account and
+# Compute API enabled.
+# See:  https://developers.google.com/compute/docs/authentication
 
 from googleapiclient.discovery import build
 from oauth2client.client import GoogleCredentials