chore: add CONTRIBUTING.md #29

add a file that describes that pull requests are welcome.
Describe how to

• setup the project locally
• run tests (pytest)
• run analysis (mypy)
• how deployment/puiblishingf works (just create a github release and a Githb workflow will publish it to PyPI)

We expect pull requests to be scoped/focussed on one thing only.
When planing larger refactoring or code changes, we ask to open a ticket first, whcih may be used to discuss the matter.
Anyway, opening a ticket to discuss issues or features are appreciated.

add anything a contributing file should include nowadays.