Add instruction on how to configure local build within the nix shell

[Townk]

This flake is great, I was able to get my config up and running pretty quickly. The one problem I have is the fact that every time I update my layout, I have to build the entire firmware to flash my device.

Is there a way to get the nix build happening in the config's directory?

Maybe even a shell command that would bootstrap a local build for these quick iterations on the layout?