Add some explanatory text to the top of the homepage

@jospoortvliet @jancborchardt @karlitschek @kelleybrooks 

There is quite a lot of text around now, which is my first concern.

![screen shot 2014-07-01 at 16 07 13](https://cloud.githubusercontent.com/assets/660805/3444908/80f45742-0132-11e4-895f-0e00ee8dae28.png)
![screen shot 2014-07-01 at 16 07 06](https://cloud.githubusercontent.com/assets/660805/3444909/8356a5c6-0132-11e4-8d4c-004efdc4bd3f.png)