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...!
One big hurdle is that many Python extension modules incorrectly link against a particular libpython instead of allowing the Python symbols to be resolved at runtime, and segfault when imported from a different Python, so in general bottles aren't interchangeable. This is usually an upstream bug AFAICT but it's common.

Thanks for checking numpy; it does seem well-behaved. That makes me less worried about falling back to homebrew/python/numpy since we can depend on it without making the parent formula depend on `:python` — but we'd have to worry about setting PYTHONPATH inside the parent formula so that a Homebrew numpy is found by system python inside the install method without user intervention.