Add Crowdin Code of Conduct to UI repository

Background

We've been using Crowdin for over 7 years without a Code of Conduct for translation contributors. During that time, we've run into multiple issues with contributors, including bullying, purposely inaccurate translations, and spam. To eliminate these headaches from here on out, we've put together a Code of Conduct that contributors must follow.

What to do

Add the Code of Conduct MD file to the root of the UI repository.