Add ID token verification helpers. (#82)

diff --git a/docs/reference/google.oauth2.rst b/docs/reference/google.oauth2.rst
index 9126549..adb9403 100644
--- a/docs/reference/google.oauth2.rst
+++ b/docs/reference/google.oauth2.rst
@@ -12,5 +12,6 @@
 .. toctree::
 
    google.oauth2.credentials
+   google.oauth2.id_token
    google.oauth2.service_account