nix: add standalone project flake

README.md

@@ -40,6 +40,23 @@ other workspaces.
 
 ## Build and install
 
+### Nix
+
+The project flake owns the Rust and GTK build dependencies. Build or run the
+locked package directly:
+
+```sh
+nix build
+nix run . -- show
+```
+
+Use `nix develop` for a shell containing Cargo, Clippy, rustfmt, and the native
+GTK dependencies. Downstream flakes can install
+`packages.${system}.default` and make this flake's `nixpkgs` input follow
+their own.
+
+### Cargo
+
 Run this from the directory containing this README:
 
 ```sh
@@ -48,8 +65,9 @@ cargo clippy --workspace --all-targets -- -D warnings
 cargo install --path . --root ~/.local --force
 ```
 
-The binary is installed as `~/.local/bin/hypr-switcher`. The accompanying
-`../hyprland.conf` starts the daemon and defines the bindings:
+The binary is installed as `~/.local/bin/hypr-switcher`. A Nix profile usually
+exposes it as `~/.nix-profile/bin/hypr-switcher`. Configure the applicable
+absolute path, then start the daemon and define the bindings:
 
 ```ini
 $switcher = ~/.local/bin/hypr-switcher