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...!
I'm :-1: for treating NumPy as resources. We really don't want to have every numpy required formulas vendoring their own numpy. It creates too many redundancies. Also for performance consideration, users may want to use their custom building numpy(either by passing options when building `homebrew-python/numpy` or by building completely themselves).

Also :+1: for `package_installed?`.