document cloud clientlibs integration (#301)

Author: liyanhui1228

README.rst

@@ -347,8 +347,8 @@ Service Integration
 -------------------
 
 Opencensus supports integration with various popular outbound services such as
-MySQL and Requests. To enable integration you will need to pass the list of
-services to census:
+SQL packages, Requests and Google Cloud client libraries. To enable integration
+services to census:	you will need to pass the list of services to census:
 
 .. code:: python
 
@@ -401,6 +401,16 @@ You can enable Requests integration by specifying ``'requests'`` to ``trace_inte
 
 .. _Requests package: https://pypi.python.org/pypi/requests
 
+Google Cloud Client Libraries
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Census can trace HTTP and gRPC requests made with the `Cloud client libraries`_.
+The request URL, method, and status will be collected.
+
+You can enable Google Cloud client libraries integration by specifying ``'google_cloud_clientlibs'`` to ``trace_integrations``.
+
+.. _Cloud client libraries: https://github.com/GoogleCloudPlatform/google-cloud-python#google-cloud-python-client
+
 ------
  Stats
 ------
@@ -515,4 +525,4 @@ Apache 2.0 - See `LICENSE <LICENSE>`__ for more information.
 Disclaimer
 ----------
 
-This is not an official Google product.
+This is not an official Google product.