libretro: update README.md

pkgs/applications/emulators/libretro/README.md

@@ -8,27 +8,25 @@ packages.
 The basic steps to add a new core are:
 
 1. Add a new core using `mkLibretroCore` function (use one of the existing
-   files as an example)
+   cores as an example)
 2. Add your new core to [`default.nix`](./default.nix) file
 3. Try to build your core with `nix-build -A libretro.<core>`
 
 ## Using RetroArch with cores
 
 To create a custom RetroArch derivation with the cores you want (instead of
-using `retroarchFull` that includes all cores), you can use `.override` like
+using `retroarch-full` that includes all cores), you can use `.withCores` like
 this:
 
 ```nix
 { pkgs, ... }:
 
 let
-  retroarchWithCores = (pkgs.retroarch.override {
-    cores = with pkgs.libretro; [
-      bsnes
-      mgba
-      quicknes
-    ];
-  });
+  retroarchWithCores = (pkgs.retroarch.withCores (cores: with cores; [
+    bsnes
+    mgba
+    quicknes
+  ]));
 in
 {
   environment.systemPackages = [
@@ -36,3 +34,5 @@ in
   ];
 }
 ```
+
+For advanced customization, see `wrapRetroArch` wrapper.