Added a note on how to test changes
Added a note on how to test changes

It's confusing for new contributors to test the client.
Added a quick note to the README explaining how to do so.

Change-Id: I2a185f04bbec7ab01fdd2ab436a5f413f33819e7

dtroyer/python-openstackclient,dtroyer/python-openstackclient,openstack/python-openstackclient,openstack/python-openstackclient