Add ID token verification helpers. (#82)

diff --git a/docs/reference/google.oauth2.id_token.rst b/docs/reference/google.oauth2.id_token.rst
new file mode 100644
index 0000000..db38b60
--- /dev/null
+++ b/docs/reference/google.oauth2.id_token.rst
@@ -0,0 +1,7 @@
+google.oauth2.id_token module
+=============================
+
+.. automodule:: google.oauth2.id_token
+    :members:
+    :inherited-members:
+    :show-inheritance: