Add best practices for making imperative code more FRP-like

This adds two new sections to the Design Guidelines:

 1. [Avoid explicit subscriptions and disposal](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/frp-best-practices/Documentation/DesignGuidelines.md#avoid-explicit-subscriptions-and-disposal)
 1. [Avoid using subjects when possible](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/frp-best-practices/Documentation/DesignGuidelines.md#avoid-using-subjects-when-possible)