That's not going to work. Here's why:
https://github.com/blog/1184-contributing-guidelines

A `contributing.md` file is specifically designed to show up when someone submits a PR. Its intention is to remind the submitter to adhere to the guidelines upon submission visibly and somewhat annoyingly :) I noticed that at times we have had to remind folks to clean up the code based on such guidelines and so, with a more visible reminder on the page, perhaps we'd be doing less work on the pulls.