Add build instructions to overall README

[janusvm]

Compared to having the keymap in a fork, the build process is a bit more complicated and will require cloning another repo and symlinking files into it.
This should be described in the README of this repo.

[janusvm]

Actually, the overall README can have overall instructions, but each individual keymap should have specific build instructions in their own README.

[janusvm]

With 678ab1e, the README(s) need to be further updated to reflect the new userspace folder.