add readme

flake.nix

@@ -42,7 +42,7 @@
       devShells.default = pkgs.mkShell {
         nativeBuildInputs = with pkgs; [
           (rust-bin.nightly.latest.default.override {extensions = ["rls"];})
-          udev
+          (pkgs.lib.optional pkgs.stdenv.isLinux udev)
           pkgconfig
           mosquitto  # For testing mqtt with mosquitto_pub
         ];

readme.md

@@ -0,0 +1,11 @@
+# DDCMQTT
+DDCMQTT is a little server program that sends DDC commands to monitors in response to
+MQTT messages. This allows you to, for example, change brightness and input from
+whatever custom scripts/hardware/control system you want.
+
+## Status
+It works, on a RPi 4, and is in daily use by the author. Code's a bit messy.
+
+## Building
+You'll need a working Rust environment, or Nix. Run `cargo build` or `nix build`
+accordingly.