[feature] Add CONTRIBUTING.md

[crenshaw-dev]

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I didn't know the project was short on dev help until the repo was archived. I'd like to stay a bit active and help if possible, since Argo CD currently depends on mux.

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What would the feature look like? How would it work? How would it change the API?

If there are regular contributors meetings or a mailing list or anything else that would be helpful for contributors, it would be good to add that to CONTRIBUTING.md. Also, if there's a way to donate, that should be documented.

I'm happy to write this up, if a maintainer can drop a quick bullet list of things the doc should include.