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...!
> For brew-packaged dependencies like NumPy, should we vendor them as resources or use a cross-tap dependency?

To dodge the question: applications in core should prefer the system Python and anything that uses the system Python should use the system numpy (and scipy) on Lion and newer if it can.

Failing that, yes, this is generally a problem. Things that end up in homebrew-python are usually there because they're somehow annoying to build, so vendoring them may be correspondingly annoying. But depending_on anything in homebrew-python for an application is bad because  homebrew/python/numpy depends on `:python` so whatever depends on it will also need to depend on `:python` instead of using the system Python, which will make it harder to bottle once bottles come to homebrew-science and disagrees with our general patterns.

Which is a shame; a NumpyRequirement that fails if numpy can't be imported and defaults to installing python/numpy for resolution would have been a nice pattern.

I am not deeply worried about supporting user optimizations but having a stable of formulas that all reimplement vendoring e.g. numpy is annoying but I guess that's where we are.

> Or should we move the numpy.rb to homebrew core?

No, this doesn't help.