Added example of a decorator in Storybook config

There was previously text on this page that recommends adding decorators to the Storybook config, but doesn't show how to do that. I was confused about this at first, so when I figured it out I decided to add it to the docs.

docs/src/pages/addons/introduction/index.md

@@ -94,7 +94,18 @@ storiesOf('Button', module)
   ));
 ```
 
-> You can call `addDecorator()` inside the story definition file as shown above. But adding it to the Storybook config file is a much better option.
+You can call `addDecorator()` inside the story definition file as shown above, but **adding it to the Storybook config file is a much better option**. That would look like this in `config.js`:
+
+```js
+import React from 'react'
+import { addDecorator } from '@storybook/react';
+
+const styles = {
+  textAlign: 'center',
+};
+const CenterDecorator = storyFn => <div style={styles}>{storyFn()}</div>;
+addDecorator(CenterDecorator);
+```
 
 ## 2. Native Addons