Add testr init in Documentation
Add testr init in Documentation

"testr init" is necessary for running testr the first time.

Change-Id: I914787c14a04476f63a7a2bbf92c6ca4fbb89b68

stackforge/monasca-api,sapcc/monasca-api,stackforge/monasca-api,openstack/monasca-api,sapcc/monasca-api,openstack/monasca-api,stackforge/monasca-api,stackforge/monasca-api,sapcc/monasca-api,openstack/monasca-api