Add macOS build instructions

[coatless]

Glancing at the getting started guide:

https://contributor.r-project.org/rdevguide/GetStart.html#building-r-and-r-packages

The focus is on Windows platforms.

If I contributed a patch to extend the documentation to incorporate macOS, would that be okay?

[llrs]

Hi @coatless as part of GSOD I will be working in this. I have access to an old MacOS, I would appreciate if later you can check the instructions still work on newer macOS. Or if you want to join the efforts and/or provide your own notes/documentation for macOS it will be helpful.

[hturner]

We have discussed this issue with @s-u and @benubah and @jaganmn will be helping Simon as he updates the official instructions for R 4.3.0 in the first place. Then we will consider what is best to add to the guide. We'd be happy to involve you @coatless if you're available to help test/review.