Fix the inappropriate database initialization in README.rst
Fix the inappropriate database initialization in README.rst

* mistral-db-manage command before first run is recommended when using
mysql as db backend.
* For SQLite, mistral-db-manage can't be used for db initialization
because of SQLite ALTER limitations.
* Correct Mistral repo URL.

Change-Id: Ib3c657637cab313e13ddbe8088788fbb9ea9c04f
Closes-Bug: #1465476

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