Add information on how to define range in comments plugin

To use the [`getComment`](https://docs.handsontable.com/pro/1.14.2/Comments.html#getComment) method we need to add a `range` for the plugin and this is what we write about range

![image](https://user-images.githubusercontent.com/10757813/31008812-ad8d19a4-a505-11e7-9b23-4b58b21733d3.png)

While we should add an example of how to set the range

demo: http://jsfiddle.net/39vk5ctf/