Add contributing guidelines

[wtimme]

As a developer, I want the project to have contributing guidelines, so that we have a common understanding of how to contribute

I suggest we add the guidelines to a file (cp. this CONTRIBUTING.md).

Topics to discuss

• Issues
  • How to create feature requests?
    • We could create Git issue templates (like this)
• Coding Guidelines
  • For formatting, I suggest we add a formatter (cp. #201)
• Best practices for Git
  • Branches
    • We use master as the main branch
    • For development, we create feature/refactoring/bugfix branches
  • Atomic commits
  • Minimum requirements for commit messages
• Pull requests
  • What needs review? Who is reviewing?
• New logic should be covered by unit tests

This list is not meant to be comprehensive. Rather, it is supposed to serve as a starting point for the discussion.