A bit of love for contribution guidelines

- [x] Fixed the broken markup.
- [x] Added TOC.
- [x] Renamed to “CONTRIBUTING”, so it would appear at GitHub's “New Pull Request” and ”New Issue“ pages like this:

![image](https://cloud.githubusercontent.com/assets/177485/3954958/9c20e914-26fc-11e4-9d61-299baf0eaec6.png)

Also, as we now have plugin API, we can add Issues section to those guidelines. Also, we can think about what to place there, both in Issues and PR sections.