Document the recommended local build workflow in a README or similar

[peterlefanulumsdaine]

This is always good practice, but would be especially helpful for the UniMath maintainers to keep this synced with UniMath upstream issues — in particular, I would be happy to prepare a PR for #17 , but first I have to figure out how to build it as intended.

[m-lindgren]

Good idea, meanwhile here's how I build the satellites using that dune supports compositional builds:

I have the satellites cloned into my UniMath clone, so it looks like this:

UniMath
├── GrpdHITs
├── largecatmodules
├── SetHITs
├── TypeTheory
└── UniMath

then from the top UniMath folder I just do dune build SetHITs.

Similarly dune build UniMath will build only UniMath, and dune build will build everything.