Documenting keg-only outside of Homebrew.

How-to-use-keg-only-outside-of-brew:
* A new piece of documentation detailing how to deal with using
keg_only formula outside of Brew itself. We’ve had a lot of questions
and issues about this recently, and the solutions aren’t as easy and
explicit to find elsewhere, so detailing it here may help reduce some
of the questions we’ve seen recently.
* I possibly whinged too much in the second section. I’m open to
criticism and being called whiny.
* Details flags, prepending the $PATH, and dealing with pkg-config.