@drashna mentioned it'd be good to have a mention of the userspace in
the QMK structure section. Rather than rewrite the docs on userspace, I
chose to link to the existing documentation.
If you run `brew install avr-gcc`, you get a version that has
compatibility issues with LUFA. I updated the getting started guide for
osx, the qmk_install setup script, and added a section to the FAQ for
folks like me who accidentally updated avr-gcc past 7.