Added more prominent note on acknowledging Astropy

@eteq @kayleanelson - looking at the website today, I realized that our request for acknowledgment is not obvious enough, and I think this little change is enough to make people notice more without changing much. Screenshot:

![screen shot 2014-02-05 at 15 38 00](https://f.cloud.github.com/assets/314716/2087878/27714d2e-8e73-11e3-8986-77c06a267607.png)

What do you think?