Code of Conduct

### Description

Project's "Code of Conduct" document is missing and should be added.

### Solution

Follow the [GitHub instructions](https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/adding-a-code-of-conduct-to-your-project).

[This template](https://github.com/probot/template/blob/master/CODE_OF_CONDUCT.md) may be a good starting point.
