Add Code of Conduct

[techdubb]

This is to revive https://github.com/jsha/blocktogether/pull/207

It contains the original file by @doeg with change requested by @jsha

[doeg]

Thanks, @h0ke! Sorry @jsha, I totally missed your comment on the previous PR. :crocodile:

[techdubb]

Funnily enough there are no section links in the docs, but if you search for Reference-style links here: https://daringfireball.net/projects/markdown/basics