Bug 1288737 - Add CONTRIBUTING.md file to Bugzilla repo

CONTRIBUTING.md

+# Contributing to Bugzilla
+
+Bugzilla is an open source project.
+
+Over the years, hundreds of people have contributed code. We appreciate your help!
+
+
+## Filing issues
+
+When filing an issue, make sure to answer these four questions:
+
+1. What version of Bugzilla are you using (`the version is on the top right-hand corner of the front page`)?
+2. What did you do?
+3. What did you expect to see?
+4. What did you see instead?
+
+General questions should go to the [support-bugzilla mailing list](https://lists.mozilla.org/listinfo/support-bugzilla) instead of our bug tracker.
+The people there will answer or ask you to file an issue if you've tripped over a bug.
+
+
+## Contributing code
+
+Please read the [tips for Developers](https://wiki.mozilla.org/Bugzilla:Developers)
+before sending patches.
+
+The gist of it is:
+
+1. Introduce yourself and don't hesitate to ask for help
+2. Download the code, make the changes you want and test them
+3. Ask a reviewer to work with you through the [review process](https://wiki.mozilla.org/Bugzilla:Review)
+
+**We gladly accept GitHub pull requests**
+
+Unless otherwise noted, the Bugzilla source files are distributed under
+the Mozilla Public License, v2.0 found in the LICENSE file.