Update README.md

Author: ForbesLindesay

packages/react-dev-utils/README.md

@@ -295,6 +295,8 @@ Returns an object with local and remote URLs for the development server. Pass th
 
 This is an alternative client for [WebpackDevServer](https://github.com/webpack/webpack-dev-server) that shows a syntax error overlay.
 
+It currently supports only Webpack 2.x
+
 ```js
 // Webpack development config
 module.exports = {