Fill out Mac section of setup guide

[BartMassey]

Right now the Mac section of Setup is pretty minimal. It should look more like the Linux section, with detailed instructions for setup and verification.

[sirhcel]

It should look more like the Linux section, with detailed instructions for setup and verification.

As far as I'm overseeing this and I can remember my first steps, there is no need to explicitly configure (and verify permissions) on macOS. USB devices are just accessible (for everyone).

So the macOS variant of this section might just not look as impressive as their Linux counterpart because there is nothing more to do in this platform.

[BartMassey]

Fair. At the very least, there should be some stuff to check to make sure that everything works.

[BartMassey]

Somebody should PR a version of rust-embedded/discovery#582. See also #21.

[BartMassey]

Also #8.