add story points explained


![storypoints_tooltip_summary](https://f.cloud.github.com/assets/831105/1176230/f25d94c6-2167-11e3-98c8-d066aca0acc7.png)
![storypoints_tooltip](https://f.cloud.github.com/assets/831105/1169012/9878321c-20aa-11e3-995c-0e94450f1ef8.png)