Add Design Guidelines and Overview docs

:warning: **NOT READY TO MERGE** :warning:

This will complete #140.

[Design Guidelines](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/design-guidelines/Documentation/DesignGuidelines.md)
[Framework Overview](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/design-guidelines/Documentation/FrameworkOverview.md)