Add macOS quick start guide

[mkincaid]

I was a bit surprised to find that the linux instructions work relatively unmodified with the standard Mac toolchain (checksums confirm that the output bin files are identical to a linux compile host). This commit creates an initial macOS quickstart page from the Linux guide, with tweaks where needed.

More modifications to come as I get the chance to do further testing with the various Mac flashing options, if I can figure out a workaround for why Dev Cube seg faults on my Mac then I'll document that, etc.