docs(readme): update with import example (#113)

Update the "getting started" section with an example importing the Compression Plugin (which is used just below).  This import (require) statement is currently not found in the README.

Not a huge deal, but could save somebody ~5 seconds of having to add that in themselves.

README.md

@@ -33,6 +33,8 @@ Then add the plugin to your `webpack` config. For example:
 **webpack.config.js**
 
 ```js
+const CompressionPlugin = require('compression-webpack-plugin');
+
 module.exports = {
   plugins: [
     new CompressionPlugin()