Add CODE_OF_CONDUCT.md

[ShaneCurcuru]

Describe the issue
Automattic is widely known for being a nice folks, but there's no code of conduct for this repo like there is for other Automattic repos.

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Look in right side github About section

Screenshots

Expected behavior
A default code of conduct should be picked up by GitHub and a link displayed in the About section. Some open source contributors won't contribute to projects without a COC.

Note: GitHub has a wide variety of default community health files they've been improving functionality of recently; Automattic could probably just set these up org-wide.
https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/creating-a-default-community-health-file

Thanks! I'm looking into newspack for a tiny local nonprofit newspaper; features look great, trying to figure out budgeting/local support.