readme: use a fully working example

Author: srid

README.md

@@ -8,20 +8,26 @@ To keep `flake.nix` smaller (eg.: going from this [91-line flake.nix](https://gi
 
 ## Usage
 
+To use haskell-flake in your Haskell projects, create a `flake.nix` containing the following:
+
 ```nix
 {
-  outputs = { self, nixpkgs, flake-parts, haskell-flake, ... }:
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+    haskell-flake.url = "github:srid/haskell-flake";
+  };
+  outputs = inputs@{ self, nixpkgs, flake-parts, ... }:
     flake-parts.lib.mkFlake { inherit self; } {
       systems = nixpkgs.lib.systems.flakeExposed;
-      imports = [
-        haskell-flake.flakeModule
-      ];
+      imports = [ inputs.haskell-flake.flakeModule ];
       perSystem = { self', pkgs, ... }: {
         haskellProjects.default = {
           root = ./.;
-          buildTools = hp: {
-            fourmolu = hp.fourmolu;
-          };
+          # buildTools = hp: { fourmolu = hp.fourmolu; };
+          # source-overrides = { };
+          # overrides = self: super: { };
+          # modifier = drv: drv;
         };
       };
     };