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.
@rlidwka already covered the contact part here