This change introduces a full Nix packaging setup with package.nix, overlay.nix, and a default.nix entrypoint, making the mpv notification script easier to consume from Nix-based systems. The package patches notify-send.lua to use the store path for libnotify's notify-send, which avoids relying on ambient PATH configuration and makes installs more reproducible. The README was also updated with Nix/NixOS usage instructions so users can wire the script into their mpv setup more easily. Practically, Nix users now get a smoother, more reliable installation path.