Add a license about code-of-conduct

[orta]

I've never seen a license on a code of conduct before, which I guess is why the CC0 1.0 makes sense.

[lazerwalker]

I adapted this from the Safe Spaces Policy I was part of writing for another Slack organization; we probably should have handled this then. I'll check with the people who helped originally write this and get back to you, but as a gut feeling CC0 1.0 makes sense to me.

[mtitolo]

CC0 1.0 makes sense to me as well 👍

[mergesort]

@cdzombak Would you care to PR and I'll merge it?

[cdzombak]

@mergesort done in #3.

Thanks, everyone!