Add Mistral service and endpoint registration to README.rst
Add Mistral service and endpoint registration to README.rst

This is a following patch of https://review.openstack.org/#/c/197851/

Change-Id: I158e68bcea97df44ce86303be6a01bb723b0f53e

dennybaa/mistral,StackStorm/mistral,openstack/mistral,dennybaa/mistral,openstack/mistral,StackStorm/mistral