Change contributing info to use markdown files directly from express repo

[crandmck]

Rather than just copy/paste duplicating the files, we should use the script to "pull down" the markdown content from:

• https://github.com/expressjs/express/blob/master/Contributing.md
• https://github.com/expressjs/express/blob/master/Collaborator-Guide.md
• https://github.com/expressjs/express/blob/master/Security.md

Into http://expressjs.com/en/resources/contributing.html.