flake: add formatter attribute in the output

README.adoc

@@ -85,6 +85,9 @@ Though, these are just templates mostly for my own purposes so it is not as usef
 * `lib` is defined from link:./lib/[my custom library].
 It's another unnecessary export but it's there.
 
+* `formatter` is the preferred formatter to be used for my Nix files.
+Nothing special here.
+
 
 === The remote repo
 

flake.nix

@@ -341,5 +341,10 @@
           description = "Basic development shell template";
         };
       };
+
+      # No amount of formatters will make this codebase nicer but it sure does
+      # feel like it does.
+      formatter =
+        forAllSystems (system: nixpkgs.legacyPackages.${system}.nixpkgs-fmt);
     };
 }