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...!
Can probably slim down the library advice and make it more black-and-white; say we won't accept them into core full stop and they might be accepted into Homebrew Python if there's a really good reason.