Documentation cleanup

This organizes content in a way that should ease people in better, and make more sense.

I wanted to combine the "When to use RAC" content with the existing README samples, but that's sort of a big effort on its own. Having it in the right place is a good first step.

**Previews:**

 * [README](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/master/README.md)
 * [Design Guidelines](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/master/Documentation/DesignGuidelines.md)
 * [Framework Overview](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/master/Documentation/FrameworkOverview.md)