[Ci-users] Added the mattermost Plugin to ci.centos.org

Hi Folks,

In response to a request (https://bugs.centos.org/view.php?id=13727) we were able to add the Mattermost plugin to CICO. If you have a mattermost instance, see the documentation on the plugin page to help with the configuration: (https://wiki.jenkins.io/display/JENKINS/Mattermost+Plugin)

We will not be setting a global configuration for this plugin, so you're responsible for each job you wish to connect up.

Cheers!

-- Brian Stinson