Add a Contributor/Member Code of Conduct

As the group grows and matures it's important to ensure everyone including current and future members feel welcome. To that end I'm planning on adding an explicit Code of Conduct to the group.

I've been researching existing Codes of Conduct used by different communities, open source organizations, and conferences. This one appears to be a mature and well-established option that is designed for adoption by third parties (like devBetter).

https://www.contributor-covenant.org/version/2/1/code_of_conduct/

In terms of implementation steps, I'm looking at the following:

- [x] Adapt the text and update it as needed for our organization and points of contact (i.e. me).
- [x] Host the content on the website in a publicly accessible URL
- [x] Update the read-me-first channel in Discord to reference the CoC by URL
- [ ] Announce the existence of the CoC and its adoption to all existing members via Discord and next coaching call

For future consideration:
- Consider requiring members to state they've read and agree to abide by the CoC as part of onboarding

Other suggestions? Leave a comment below.