Add CONTRIBUTING.md

[synalice]

It's not the best one, but might be a good place to start.

[un-versed]

Nice! I was missing this...

What about commit messages? Should we have a pattern?

Personally I like to use https://www.conventionalcommits.org/en/v1.0.0/

[synalice]

@un-versed, absolutely agree. They are very nice and already seem to be the standard for the repository.

Although I'm not sure if it's necessary to add this into CONTRIBUTING.md since all PRs when merged get squashed into single properly named commit.

@lucacasonato, what do you think?

[iuioiua]

Closing this as Deno repos mainly refer to a central Contributing page. We're adding the link to that page in the README in #1233.