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.
Do you have any direct link you can provide within this doc that allows for reporting harassment/abuses? Might make things easier than having to follow through the code of conduct and then look through from there. 

Or it's possible you want people to read through the CoC before reporting, in which case you might want to make that clear here just in case someone has landed at the wrong .md