Add commit message prefix to instructions.

As was discussed in the Oslo meeting 2014-10-24, we're going to
ask committers to prefix commit messages with the name of the lib
their spec relates to.  See the discussion starting around 16:20
in the meeting minutes from that day:
http://eavesdrop.openstack.org/meetings/oslo/2014/oslo.2014-10-24-16.07.log.html

Change-Id: I944a3006850d518cff1497d2de213d1584471ef1