added CONTRIBUTING file

I noticed you often have to repeat that PR's should be send to `develop` branch. Instead of doing that, I've created a `CONTRIBUTING.md` which allows you to settle it once and for all, to redirect PR's to the right branch.

You can read more information about adding a CONTRIBUTING file [here](https://help.github.com/articles/how-do-i-set-up-guidelines-for-contributors).

PS: I don't think this PR is worth to send to `develop` branch :)