Add more incentive to write a good PR description

Pivotal stories as PR descriptions is not ideal as most stories have either too much, or too little information. This encourages developers to write a proper description of their changes.

![screenshot 2014-08-21 09 24 47](https://cloud.githubusercontent.com/assets/293672/3993474/0ad57c0a-290d-11e4-9ac4-f9756df72390.png)