Add documentation about using this template with an existing project

[drewoldag]

It would be good to include some information about what to expect if moving from a setup.py file to pyproject.toml file.
And explain what directory structure is "necessary" for the various components to work together. i.e. pylint expects there to be a ./src directory for linting.

[drewoldag]

Would be good to add a line about the dynamic "version" not playing nicely if the user has a predefined, hardcoded "version" in the pyproject. toml.