Document the release process

[darosior]

For my future self, and potential other future maintainers.

Some notes i've got for now:

• Update CHANGELOG.md
• Bump version in pyproject.toml
• Make sure the minimum supported Python version in pyproject.toml is correct
• Make sure all new interfaces are documented
• Git tag
• Github release
• py -m build
• twine upload to testpypi
• twine upload to pypi