Added Contributing.md to help stave off unwanted feature requests

Github [automatically links to the `CONTRIBUTING.md` document](https://github.com/blog/1184-contributing-guidelines) at the top of the create issue page.  This provides a way to gently guide armchair product managers in the right direction.

Here's a stub contributing file that might help stave off out of scope feature suggestions like [the one I just submitted](https://github.com/heroku/hk/issues/9) earlier today.

Thanks! :smile: