add pre-commit configuration

based on isort, black, blackdoc, flake8 and mypy

This MR is a follow-up on the discussion we had in !27 (comment 34579)

I also added short installation and contribution instructions to the README.md.

@daniel-eggert and @nils-brinckmann, it would be great if you could give it a try and tell me, if this is a working solution for you.