Changed the README.rst
Changed the README.rst

Some useful links were missing from the README file which
would be useful for the first timers. So, adding those links
into the file.

Change-Id: Iac89fed65190084878aeac93987cd857793c9424
Partial-Implements: blueprint mistral-python-client-docs

openstack/python-mistralclient,StackStorm/python-mistralclient,StackStorm/python-mistralclient,openstack/python-mistralclient