[doc] specify target extension

README.md

@@ -84,7 +84,8 @@ entry: {
 
 plugins: [
     new InjectAssetsPlugin({
-        // replace assets in the service worker and in the app script
+        // replace assets in the service worker and in the app script;
+        // since there would also be an app css asset, we have to specify the type here
         targets: ['serviceWorker', 'app|js']
     })
 ]