devel: Created CONTRIBUTING.md

Initial commit of contributor guidelines.

Includes:
- Simple directions for where to add issues, questions or feature requests
- A short coding guideline
- An extensive commit guideline

Signed-off-by: Sebastian Niemann <niemann@sra.uni-hannover.de>
Merge pull request #3 from SebastianNiemann/develop

devel: Created CONTRIBUTING.md