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>`.
diff --git a/google/oauth2/id_token.py b/google/oauth2/id_token.py
index fa96fc0..208ab62 100644
--- a/google/oauth2/id_token.py
+++ b/google/oauth2/id_token.py
@@ -12,7 +12,51 @@
# See the License for the specific language governing permissions and
# limitations under the License.
-"""Google ID Token helpers."""
+"""Google ID Token helpers.
+
+Provides support for verifying `OpenID Connect ID Tokens`_, especially ones
+generated by Google infrastructure.
+
+To parse and verify an ID Token issued by Google's OAuth 2.0 authorization
+server use :func:`verify_oauth2_token`. To verify an ID Token issued by
+Firebase, use :func:`verify_firebase_token`.
+
+A general purpose ID Token verifier is available as :func:`verify_token`.
+
+Example::
+
+ from google.oauth2 import id_token
+ from google.auth.transport import requests
+
+ request = requests.Request()
+
+ id_info = id_token.verify_oauth2_token(
+ token, request, 'my-client-id.example.com')
+
+ if id_info['iss'] != 'https://accounts.google.com':
+ raise ValueError('Wrong issuer.')
+
+ userid = id_info['sub']
+
+By default, this will re-fetch certificates for each verification. Because
+Google's public keys are only changed infrequently (on the order of once per
+day), you may wish to take advantage of caching to reduce latency and the
+potential for network errors. This can be accomplished using an external
+library like `CacheControl`_ to create a cache-aware
+:class:`google.auth.transport.Request`::
+
+ import cachecontrol
+ import google.auth.transport.requests
+ import requests
+
+ session = requests.session()
+ cached_session = cachecontrol.CacheControl(session)
+ request = google.auth.transport.requests.Request(session=cached_session)
+
+.. _OpenID Connect ID Token:
+ http://openid.net/specs/openid-connect-core-1_0.html#IDToken
+.. _CacheControl: https://cachecontrol.readthedocs.io
+"""
import json
