add usage example to readme

Signed-off-by: Mark Sagi-Kazar <[email protected]>

README.md

@@ -4,3 +4,28 @@
 
 <!-- Remove this if you don't use github actions -->
 ![Build and populate cache](https://github.com/dagger/nix/workflows/Build%20and%20populate%20cache/badge.svg)
+
+## Usage
+
+### As flake
+
+```nix
+{
+  inputs = {
+    nixpkgs.url = "nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    dagger.url = "github:dagger/nix";
+    dagger.inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, dagger, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in {
+        devShell = pkgs.mkShell {
+          buildInputs = [ dagger.packages.dagger ];
+        };
+      });
+}
+```