Updated Watcher doc to mention Tempest tests

The Watcher Tempest tests are only mentioned inside a README.rst.
They are now part of the main documentation.

Change-Id: Ieca85dc7f7307b45e4b99af4a4600a8c2d2b59d7
Closes-Bug: #1536993

doc/source/dev/testing.rst

@@ -0,0 +1,50 @@
+..
+      Except where otherwise noted, this document is licensed under Creative
+      Commons Attribution 3.0 License.  You can view the license at:
+
+          https://creativecommons.org/licenses/by/3.0/
+
+=======
+Testing
+=======
+
+.. _unit_tests:
+
+Unit tests
+==========
+
+All unit tests should be run using `tox`_. To run the same unit tests that are
+executing onto `Gerrit`_ which includes ``py34``, ``py27`` and ``pep8``, you
+can issue the following command::
+
+    $ workon watcher
+    (watcher) $ pip install tox
+    (watcher) $ cd watcher
+    (watcher) $ tox
+
+If you want to only run one of the aforementioned, you can then issue one of
+the following::
+
+    $ workon watcher
+    (watcher) $ tox -e py34
+    (watcher) $ tox -e py27
+    (watcher) $ tox -e pep8
+
+.. _tox: https://tox.readthedocs.org/
+.. _Gerrit: http://review.openstack.org/
+
+You may pass options to the test programs using positional arguments. To run a
+specific unit test, you can pass extra options to `os-testr`_ after putting
+the ``--`` separator. So using the ``-r`` option followed by a regex string,
+you can run the desired test::
+
+    $ workon watcher
+    (watcher) $ tox -e py27 -- -r watcher.tests.api
+
+.. _os-testr: http://docs.openstack.org/developer/os-testr/
+
+When you're done, deactivate the virtualenv::
+
+    $ deactivate
+
+.. include:: ../../../watcher_tempest_plugin/README.rst