Add google.oauth2.flow - utility for doing OAuth 2.0 Authorization Flow (#100)

diff --git a/docs/reference/google.oauth2.rst b/docs/reference/google.oauth2.rst
index adb9403..5dc2406 100644
--- a/docs/reference/google.oauth2.rst
+++ b/docs/reference/google.oauth2.rst
@@ -12,6 +12,7 @@
 .. toctree::
 
    google.oauth2.credentials
+   google.oauth2.flow
    google.oauth2.id_token
    google.oauth2.service_account