Document Contibuting / Code Style Guidelines

[fengelniederhammer]

There is a lot of untested code with huge React components that contain both, display and data processing logic. It would be good to split those, such that the data processing can be tested independently of the display logic.

The proposal to document the target state is:

• Write a contributing.md or similar which contains code style guidelines.
• Refactor a component / widget such that it serves as an example.