CODE_OF_CONDUCT.md file missing

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 reach out to Mozilla-GitHub-Standards+CoC@mozilla.com.

_(Message COC001)_

Start by comparing the repository root with the linked Mozilla CODE_OF_CONDUCT.md template. Add the missing root-level file using the required Community Participation Guidelines and How to Report text, then verify that the file is present and that the required text is unchanged; treat the optional Project Specific Etiquette section according to the project’s needs.

索引モデルが issue の本文から書いたものです。