Add CoC linkback to all open repos

[asmeurer]

We need to add a CODE_OF_CONDUCT.md file to all repos that points back to this one. This repo will still be shown for any repo that doesn't have this file, but the visibility on GitHub of the "default" CoC isn't as great as just having that file in the root of the repo.

[asmeurer]

OK, going to try doing this now.

[asmeurer]

This is what it looks like Quansight-Labs/ndindex#112. Going to do it now for every repo 🤞

[asmeurer]

Actually, maybe not a smart idea to do this at the end of the day :). Going to do the full thing tomorrow or Friday.