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...!
Or should we move the numpy.rb to homebrew core? We didn't do it before because it has cross-tap dependencies. But as for now the only cross-tap dependency is *optional*. @mikemcquaid