Add anti-harassment policy as CODE_OF_CONDUCT.md

[jeancochrane]

Overview

Add the DataMade Anti-Harassment Policy as a CODE_OF_CONDUCT.md to this repo.

Closes #13.

Testing instructions

• View the Markdown preview of the new README: https://github.com/datamade/how-to/blob/feature/jfc/code-of-conduct/README.md
• View the Markdown preview of the new CoC: https://github.com/datamade/how-to/blob/feature/jfc/code-of-conduct/CODE_OF_CONDUCT.md