Document the release procedure (PyPI and conda-forge)

[Zeitsperre]

The methods we use to tag a new release should be clearly stated in the contributing documentation. I don't think we do anything elaborate, so we can likely borrow directly from documentation of existing/related projects (e.g. https://github.com/Ouranosinc/xclim/issues/813).

@cjauvin Is there anything that you do that differs from the following when creating a release for PyPI?:

• Create a PR to tag a new version
  • Bump the version number (bumpversion minor)
  • Update history file
  • merge (→ master/main)
• git checkout master
• git tag v<major>.<minor>.<patch>
• git push --tags
• python setup.py sdist bdist_wheel
• twine upload dist/*

[cjauvin]

@Zeitsperre That's exactly my list.

[Zeitsperre]

This has been dealt with in https://github.com/Ouranosinc/cookiecutter-pypackage/pull/10. Once this has been merged, we can update the docs here via cruft.