Add urllib3 transport (#14)

diff --git a/docs/reference/google.auth.rst b/docs/reference/google.auth.rst
index 0f147e9..ef0d689 100644
--- a/docs/reference/google.auth.rst
+++ b/docs/reference/google.auth.rst
@@ -11,6 +11,7 @@
 
 .. toctree::
 
+    google.auth.compute_engine
     google.auth.transport
 
 Submodules