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.
Thanks! Sounds great :+1:. I couldn't really come up with a decent description for the file so I just plonked the most obvious thing I could think of in there temporarily. Your suggestion sounds great.