Reapply lost change from gatsby crossover

See: storybook-eol/storybooks.github.io#8

pages/docs/react-storybook/configurations/custom-webpack-config/index.md

@@ -2,6 +2,7 @@
 id: 'custom-webpack-config'
 title: 'Custom Webpack Config'
 ---
+**NOTE: Storybook doesn't currently support webpack 2, so write your storybook webpack config to be webpack 1.x-compatable.**
 
 The default Webpack config of React Storybook is usually well balanced for a medium-size React project (specially created with [Create React App](https://github.com/facebookincubator/create-react-app)) or a library. But if you already have your own Webpack setup, that's not useable.