Updates to Code of Conduct

The existing Code of Conduct is modeled after CoC's used at conferences within the US. PWL is a loose confederation of meet-ups with an international reach and has slightly different needs. The updated CoC still explicitly states that PWL meetups and online communities should be harassment-free environments, but that it is up to the local organizers of live events to maintain that environment.
For the IRC note, we should add `If you are experiencing harassment on or have concerns about content within the GitHub repo, on the IRC channel (#paperswelove), or [PapersWeLove.org](http://paperswelove.org) please contact`.

We'll add the note about the channel on the README itself.