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...!
> which will make it harder to bottle once bottles come to homebrew-science and disagrees with our general patterns.

As an newbie, can you explain to me why the bottle cannot work for both system python and homebrew python? Why we cannot create an universal bottle? Thanks. Also as for my test going, the numpy installed by `homebrew-python/numpy.rb` with brewed python works well in system python.