Adds FAQ item on customizing paths for sources and binaries, fixes typo

Nathan Ringo requested to merge nringo-doc-additions into master

It's my first time writing Texinfo, so if there's some additional linter or something to run, I can do so!

This adds a short FAQ item describing the small build script I use for building binaries as part of a larger (mostly non-CL) project. It hopefully conforms to best practices; if not, I'll change it such that it does.

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