arx/contributing.md

We appreciate any form of contribution! A few notes:

• When you have a general question about data anonymization or ARX, please do not create an issue but send us an email.
• When creating a pull request, try to make your changes as small as possible.
• When implementing a new feature or reporting a bug, try to include tests where possible and sensible.
• Try to follow our coding style and to minimize style changes in existing code.