Add editor section in readme

Author: alexanderchr

README.md

@@ -83,3 +83,11 @@ Trigger a notification every time.  Call it "noisy-mode".
 ```js
 new WebpackNotifierPlugin({alwaysNotify: true});
 ```
+
+### Editor
+
+Runs the specified editor when the notification is clicked.
+
+```js
+new WebpackNotifierPlugin({ editor: 'atom' });
+```