Adding description of "borrowed" vendor code.

See googleapis#400 for context.

CONTRIBUTING.rst

@@ -285,6 +285,33 @@ should either be:
   ``EXTRA_TOX_ENVS``. This value is unencrypted in ``gcloud-python-wheels``
   to make ongoing maintenance easier.
 
+Shared Code with External Projects
+----------------------------------
+
+In order to enable high-quality HTTP transfer of large data (for Cloud
+Storage), we have temporarily included some code from the
+`apitools <https://github.com/craigcitro/apitools>`__  library.
+
+We have chosen to partially include it, rather than include it as
+a dependency because
+
+- The library is not yet included on PyPI.
+- The library's ``protorpc`` dependency is not Python 3 friendly, so
+  would block us from Python 3 support if fully included.
+
+The included code in lives in the
+`_gcloud_vendor <https://github.com/GoogleCloudPlatform/gcloud-python/tree/master/_gcloud_vendor>`__
+directory. It is a snapshot of the ``e5a5c36e24926310712d20b93b4cdd02424a81f5``
+commit from the main project imported in
+``4c27079cf6d7f9814b36cfd16f3402455f768094``. In addition to the raw import,
+we have customized (e.g. rewriting imports) for our library:
+
+- ``334961054d875641d150eec4d6938f6f824ea655``
+- ``565750ee7d19742b520dd62e2a4ff38325987284``
+- ``67b06019549a4db8168ff4c5171c9d701ac94a15``
+- ``f4a53ee64fad5f3d7f29a0341e6a72a060edfcc2``
+
+
 Contributor License Agreements
 ------------------------------