Added note to install openstackclient

Change-Id: Idcd9ef4e7a10ebbd8b68e7320680f503dfc166a9
Closes-Bug: #1483384

doc/source/developing.rst

@@ -87,3 +87,20 @@ create HTML docs, run the following:
     $ tox -e docs
 
 The resultant HTML will be the ``doc/build/html`` directory.
+
+Testing new code
+----------------
+
+If a developer wants to test new code (feature, command or option) that
+they have written, OpenStackClient may be installed from source by running
+the following commands in the base directory of the project:
+
+.. code-block:: bash
+
+   $ python setup.py develop
+
+or
+
+.. code-block:: bash
+
+   $ pip install -e