add note about options in README

Author: sokra

README.md

@@ -46,6 +46,8 @@ const MiniCssExtractPlugin = require("mini-css-extract-plugin");
 module.exports = {
   plugins: [
     new MiniCssExtractPlugin({
+      // Options similar to the same options in webpackOptions.output
+      // both options are optional
       filename: "[name].css",
       chunkFilename: "[id].css"
     })