Add Mozilla Code of Conduct (#1656)

Fixes #1655


As of January 1 2019, Mozilla requires that all GitHub projects include this [CODE_OF_CONDUCT.md](https://github.com/mozilla/repo-templates/blob/master/templates/CODE_OF_CONDUCT.md) file in the project root. The file has two parts:

1. Required Text - All text under the headings *Community Participation Guidelines and How to Report*, are required, and should not be altered.
2. Optional Text - The Project Specific Etiquette heading provides a space to speak more specifically about ways people can work effectively and inclusively together. Some examples of those can be found on the [Firefox Debugger](https://github.com/devtools-html/debugger.html/blob/master/CODE_OF_CONDUCT.md) project, and [Common Voice](https://github.com/mozilla/voice-web/blob/master/CODE_OF_CONDUCT.md). (The optional part is commented out in the [raw template file](https://raw.githubusercontent.com/mozilla/repo-templates/blob/master/templates/CODE_OF_CONDUCT.md), and will not be visible until you modify and uncomment that part.)

If you have any questions about this file, or Code of Conduct policies and procedures, please see [Mozilla-GitHub-Standards](https://wiki.mozilla.org/GitHub/Repository_Requirements) or email Mozilla-GitHub-Standards+CoC@mozilla.com.

_(Message COC002)_

CODE_OF_CONDUCT.md

@@ -0,0 +1,8 @@
+# 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.