docs: Python for formula authors

Begin to document the current consensus about correct Python behavior in Homebrew formulas. The libraries advice is a little more experimental than the applications advice but I've started to experiment with it in homebrew-python and nobody's complained yet...!
You could imagine something like this but I think you'd want to add logic to the bottling process to do it and Mike is ideologically (and I think justly) opposed to bottling more than one configuration so I think probably not, in practice.