Add urllib3 transport (#14)

diff --git a/docs/reference/google.auth.transport.rst b/docs/reference/google.auth.transport.rst
index 88b427c..7066334 100644
--- a/docs/reference/google.auth.transport.rst
+++ b/docs/reference/google.auth.transport.rst
@@ -6,3 +6,10 @@
     :undoc-members:
     :show-inheritance:
 
+Submodules
+----------
+
+.. toctree::
+
+   google.auth.transport.urllib3
+