[WIP] Add Code of Conduct

I may be stealing some ideas from https://www.hackerschool.com/manual#sub-sec-social-rules

I think we need a reference to https://www.python.org/psf/codeofconduct also.

This is a work-in-progress.
This suggestion seems odd without a suggested alternative (constructive criticism/comparisons).  Maybe we should instead say "keep criticism positive and helpful"?

The things I had in mind were statements like "I use Django now because web2py sucks" which is an unhelpful statement and might alienate web2py contributors in the audience.