Improve documentation around ID Tokens (#224)

diff --git a/docs/index.rst b/docs/index.rst
index 1b5f5d6..af2c804 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -14,6 +14,7 @@
 
 - Support for Google :func:`Application Default Credentials <google.auth.default>`.
 - Support for signing and verifying :mod:`JWTs <google.auth.jwt>`.
+- Support for verifying and decoding :mod:`ID Tokens <google.oauth2.id_token>`.
 - Support for Google :mod:`Service Account credentials <google.oauth2.service_account>`.
 - Support for :mod:`Google Compute Engine credentials <google.auth.compute_engine>`.
 - Support for :mod:`Google App Engine standard credentials <google.auth.app_engine>`.