Add a pre-commit config file

and document it briefly in the README file.

This comes with a few checkers:

• flake8
• trailing-whitespace
• check-json
• check-yaml
• codespell

Also include commented checkers:

• black: disabled since we are not ready for this yet,
• pydocstyle: disabled since we have way too many pydoctyle errors for now...

See https://pre-commit.com/ for more details.

Related to T1881.