improve readme for people who are just getting started

[petergaultney]

I put some stuff in the README, but also made the build work on Mac by creating a temp directory from scratch rather than using a hardcoded one that only would work on Linux.

I didn't put a ton of thought into where this stuff should go in the README. I'm also not sure if the Arduino software is even required at this point - maybe we should note that it isn't if it's not?

[reyemxela]

Hey, thanks for the PR! Good catches on both the readme submodule instructions as well as mktemp. The build script was half for my own use for building/testing locally and half for the github workflow, so I hadn't worried a ton about portability before.

One thing I noticed when trying the updated build script locally: now that it's using set -u, I get an unbound variable error for the $2 on line 44. Which is fair, because that's exactly what it is. Looks like changing that to ${2:-} would fix it.

(also looks like something's up with the arduino-cli install script/their hosted files, and that's what caused the checks to fail when I tried just now)