Readme should point to Openstack, not travis-ci

The readme, leftover from when this was teeth-agent, was still pointing
at the travis-ci build. I removed that build status and added a link to
Openstack How to Contribute wiki page, so people visiting the mirror on
Github know not to create pull requests.

Change-Id: I8a82eb0e04cc35425f7856b16568f191c4c54770
Merge "Readme should point to Openstack, not travis-ci"