Add note for unreliable in README.rst
Add note for unreliable in README.rst

Many new comers run into trouble when their local mongodb
doesn't enable replica set. This patch just add a note for
that in README.rst.

Change-Id: Iee60bec1b009d482eb0efe99c4b1f09798be2c3c

openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar