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:
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