Add README.mistral.conf doc in etc directory
Add README.mistral.conf doc in etc directory

Currently, the mistral.conf can be generated by tox, So add the
README.mistral.conf is useful.

Change-Id: I657b0d193a8f53dd86ed7b5bf228310eddbecd75

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