Add CONTRIBUTING.md file

[aik099]

The CONTRIBUTING.md file can be very useful for users, that never used library before and are willing to contribute. Some examples:

• https://github.com/qa-tools/qa-tools/blob/develop/CONTRIBUTING.md

[aik099]

After #89 the coding standard of QA-Tools library is also used here, so we can copy-paste as-is all instructions to CS checks as well.