Add action to publish

[sabinem]

This PR adds 2 new github workflows:

• one workflow to publish the package on Pypi: it is triggered by github releases
• one workflow that is manually triggered to publish on test-Pypi.

Besides these changes:

• the version is now derived automatically in the __init__.py file, so that it only needs to be set in the pyproject.toml file
• the readme got a section on how to do a release.

[cmdoret]

Great, thank you! Just two tiny comments / questions