Add more details to the Collaborator Guide

[hacksparrow]

As suggested by @dougwilson on Express TC meeting 11-16-2016, we should add more details to the existing Collaborator Guide, focussed around code and other convention that must be followed when opening a PR.

This will help to focus PR discussions on implementations rather than on the conventions that should have been followed.

[crandmck]

Note that we also have https://github.com/expressjs/express/blob/master/Contributing.md. The distinction between these two is not immediately apparent to me...

Also, the content of these two files is duplicated in http://expressjs.com/en/resources/contributing.html.
I opened expressjs/expressjs.com#734 so we can just "pull down" the content of these files, rather than duplicating them. Then, we we update the md files in the express repo, when we run the script, they'll get updated in the docs as well.

[crandmck]

Issue moved to expressjs/expressjs.com #765 via ZenHub