factorio: give advice on how to retain source tarball (#350475)

pkgs/by-name/fa/factorio/package.nix

@@ -70,6 +70,15 @@ let
       nix-prefetch-url file://\''$HOME/Downloads/factorio_\''${releaseType}_x64_\''${version}.tar.xz --name factorio_\''${releaseType}_x64-\''${version}.tar.xz
 
     Note the ultimate "_" is replaced with "-" in the --name arg!
+
+    If you go this route you might want to tell Nix to explicitly hold on to the
+    source tarball. Otherwise it could get GC'd from the Nix store and you'd
+    have to redownload it next time the package wants to rebuild to use a newer
+    dependency. E.g. if you're using NixOS:
+
+      system.extraDependencies = [
+        factorio.src
+      ];
   '';
 
   desktopItem = makeDesktopItem {