Update README.md

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

README.md

@@ -11,6 +11,30 @@ inputs.flake-compat = {
 };
 ```
 
+Example in a `flake.nix` file:
+
+```nix
+{
+  description = "My first flake";
+
+  inputs.nixpkgs.url = "nixpkgs/nixos-unstable";
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+  inputs.flake-compat = {
+    url = "github:edolstra/flake-compat";
+    flake = false;
+  };
+
+  outputs = { self, nixpkgs, flake-utils, flake-compat }:
+    flake-utils.lib.eachDefaultSystem
+      (system:
+        let pkgs = nixpkgs.legacyPackages.${system}; in
+        {
+          devShell = pkgs.mkShell { buildInputs = [ pkgs.hello pkgs.cowsay ]; };
+        }
+      );
+}
+```
+
 Afterwards, create a `default.nix` file containing the following:
 
 ```nix