Add Contribution guidelines

[philsturgeon]

More than just our checklist, why it’s "better" than what already exists, or at least how it differs and what the uses cases are.

Too many people talk about "handlers" and "formatters" and I need a ELI5 answer.

[philsturgeon]

I'd suggest that this can come back later, we don't want to try and grow too much more, and will have to suffer more moans of "But they;re elitists who dont let people in!" instead of "They're elitists who are trying to steal everything!"