Setup docs need small update

Hi! Saw Wrike's blog post on the integration you made, and I noticed a few things in the readme that could use some updating.
Regarding this line, it looks like GitHub's Add Webhook form has a few more options. Could you perhaps add some detail here to make it more clear to a newbie what they might want to select or leave alone, @emiliorcueto?

![screen shot 2014-07-28 at 1 47 13 pm](https://cloud.githubusercontent.com/assets/414839/3724547/5be07852-167f-11e4-8cbc-a9c79c1fe576.png)