Add Design Guidelines and Overview docs

This completes #140 and #308.

- [x] [Design Guidelines](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/design-guidelines/Documentation/DesignGuidelines.md)
- [x] [Framework Overview](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/design-guidelines/Documentation/FrameworkOverview.md)