[fenix] Add Mozilla Code of Conduct file

Fixes https://github.com/mozilla-mobile/fenix/issues/1275.

_(Message COC002)_

CODE_OF_CONDUCT.md

@@ -0,0 +1,15 @@
+# Community Participation Guidelines
+
+This repository is governed by Mozilla's code of conduct and etiquette guidelines. 
+For more details, please read the
+[Mozilla Community Participation Guidelines](https://www.mozilla.org/about/governance/policies/participation/). 
+
+## How to Report
+For more information on how to report violations of the Community Participation Guidelines, please read our '[How to Report](https://www.mozilla.org/about/governance/policies/participation/reporting/)' page.
+
+<!--
+## Project Specific Etiquette
+
+In some cases, there will be additional project etiquette i.e.: (https://bugzilla.mozilla.org/page.cgi?id=etiquette.html).
+Please update for your project.
+-->