Adds nix-darwin instructions to the README

[jkachmar]

Addresses macOS-specific concerns within #96.

Overview

This PR documents how lorri may be installed and maintained as a system service using nix-darwin, in a similar manner to how it's managed with NixOS or home-manager on Linux.

Checklist

• [x] Updated the documentation (code documentation, command help, ...)
• [ ] Tested the change (unit or integration tests)
• [ ] Amended the changelog in release.nix (see release.nix for instructions)

[Profpatsch]

Thank you!