README: Add installation instructions for Fedora 18

Hello!

I've been playing around with SimpleCV on a Fedora system and thought it would be cool to send back instructions for anybody wandering by. These are based off of the Ubuntu dependencies which aren't very different aside from a few packages.

I've done capturing from the camera, and have run some of the [image segmentation](http://examples.simplecv.org/en/latest/examples/segment.html) examples with success.

cheers