Provide clearer instructions on how onboarding projects should adopt the CoC

[tobie]

We should be more specific about what adopting our CoC actually means to onboarding projects.

Part of this is covered in the upcoming CoC policy and processes, but not all of it is.

Typically, I don't think we ever formally decided whether projects should have a copy of the CoC in their repo (with the risk of then getting out of date), or whether the CoC file should just redirect to the foundation's CoC.

(This issue was prompted by openjs-foundation/project-status#67.)

[ljharb]

I think it's up to the project - but what seems to work best is a file (in an org's .github repo, if applicable) with a link in it that points to the foundation's CoC.