text: Title: Configure pre-commit for this repository and update documentation
  Body: In order to help new contributors it could be nice to add [pre-commit](https://pre-commit.com/) hooks to the repository with the following checks:
  - flake8
  - isort
  - ...?
  
  The [CONTRIBUTING.md file](https://github.com/adbar/trafilatura/blob/master/CONTRIBUTING.md) could get updated accordingly.