Add example with -a arm64 to rig add docs?

[batpigandme]

The downside of doing this is that it's platform (macOS) specific. However, given the default architecture will be x86_64 even on arm64 Macs (which makes sense, given it's in line with R distributions), it'd be nice to have the -a/--arch option be more visible in the docs/help examples.

I'd been doing, e.g. rig add 4.2-arm64, not realizing you could use devel and release with -a arm64 until I saw you do so in your demo. Basically: I think it's worth highlighting because it's handy!

[gaborcsardi]

Help is already OS specific. OTOH, I'll make arm64 the default....