docs: update ExtractTextPlugin v2x setup docs (#229)

Source: https://github.com/webpack-contrib/extract-text-webpack-plugin#usage

Otherwise breaking change error will be thrown, e.g.

```
fallbackLoader option has been deprecated - replace with "fallback"
```

README.md

@@ -210,8 +210,8 @@ Setup:
     {
       test: /\.css$/,
       loader: ExtractTextPlugin.extract({
-          notExtractLoader: 'style-loader',
-          loader: 'css?modules&importLoaders=1&localIdentName=[path]___[name]__[local]___[hash:base64:5]!resolve-url!postcss',
+          fallback: 'style-loader',
+          use: 'css?modules&importLoaders=1&localIdentName=[path]___[name]__[local]___[hash:base64:5]!resolve-url!postcss',
       }),
     }
     ```