RFC: contributing.md

why: https://medium.com/node-js-javascript/codes-of-conduct-82ab2d88112d

Still work in progress, but I whipped this up for now.

Once this is done(ish) and reviewed I (or we) __will__ be applying this to ever single repo under jshttp & pillarjs, as well as active expressjs repos and express / expressjs.com themselves.


Note: this will be squashed.
Food for thought: what are you trying to convey by using the 'positively'? While I understand the intention of using the word, I think there more specific ways to say what you'd like while being direct about it. There's nothing wrong with saying what is expected of contributors to the project.  Managing expectations is :+1: 

I've been looking at this CoC and I'm a big fan of the way they've shared what they're aiming for in contribution in [section 1](http://citizencodeofconduct.org/)