Add urllib3 transport (#14)

diff --git a/docs/reference/google.auth.transport.urllib3.rst b/docs/reference/google.auth.transport.urllib3.rst
new file mode 100644
index 0000000..32de225
--- /dev/null
+++ b/docs/reference/google.auth.transport.urllib3.rst
@@ -0,0 +1,7 @@
+google.auth.transport.urllib3 module
+====================================
+
+.. automodule:: google.auth.transport.urllib3
+    :members:
+    :undoc-members:
+    :show-inheritance: