add contributing guide

### What is the problem / feature ?

No documented contribution guidelines

### How did it get fixed / implemented ?

Added a `CONTRIBUTING.md` so that github will pick it up as [contribution guidelines](https://github.com/blog/1184-contributing-guidelines)

### How can someone test / see it ?

See that when you open a pull request, it displays a header showing you contribution guidlines.

*Here is a cute animal picture for your troubles...*

![turtle3](https://cloud.githubusercontent.com/assets/824194/4509224/549e79c2-4b1d-11e4-9a2a-282b40af4997.jpg)