Add Nix flake examples

nix/flakes/simple/flake.nix

@@ -0,0 +1,20 @@
+{
+  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+
+  outputs =
+    inputs:
+    let
+      system = "x86_64-linux";
+      pkgs = import inputs.nixpkgs { inherit system; };
+    in
+    {
+      devShells.${system}.default = pkgs.mkShell {
+        packages = with pkgs; [
+          go
+          php
+          phpPackages.composer
+          tailwindcss
+        ];
+      };
+    };
+}