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...!
Going to pass on commenting on this in the documentation but I think a NumpyRequirement that falls back to homebrew-python's numpy could be a reasonable solution for homebrew-science; drafting one is on my to-do list.