doc: add missing import (#43)

Author: thijstriemstra

README.md

@@ -31,6 +31,7 @@ Then add the plugin to your `webpack` configuration. For example:
 **webpack.config.js**
 
 ```js
+const MiniCssExtractPlugin = require('mini-css-extract-plugin');
 const CssMinimizerPlugin = require('css-minimizer-webpack-plugin');
 
 module.exports = {