CONTRIBUTING.md link gets a 404 on herokuapp

Created by: luilver

Briefing

It should link to github instead of herokuapp or replace CONTRIBUTING.md by /contributing if the idea is to remain within herokuapp.

Steps to reproduce

1. Visit Administate's Home
2. Go to section Contribute
3. Click on "Please see CONTRIBUTING.md."
4. Get a "The page you were looking for doesn't exist." error