Add contribution file

This will show an information to people before performing a pull request or opening an issue, and inform them about our contribution guidelines.

Pro:
- Users get informed that there are multiple repositories.
- Potential developers get informed about our contributing agreement.
- Users know that they should contribute the translations on transifex.

Example:
![0pgi3icoigij](https://f.cloud.github.com/assets/878997/60653/682d40ee-5c19-11e2-9614-77731e6b6309.png)

Or check it out here: https://github.com/LukasReschke/core/issues/new

@karlitschek What's your opinion on this?