some additions and clarifications for Mac & Linux installation

[ChristopherA]

👆 @gorazdko

& all: We need a better way to specify these dependencies for Mac vs. Linux.

[shannona]

I personally think listing the apt and brew commands as we do now (or in a few cases curl) is a pretty good methodology. In general, I don't think installers want to know the dependencies exactly, just how to install things so they work.

(And Mac and Linux are clearly listed in different sections, which might not be obvious from the commit.)