Describe good signal naming conventions

Part of #304. Completes #308.

[Rendered Markdown](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/signal-naming/Documentation/DesignGuidelines.md#use-descriptive-declarations-for-methods-and-properties-that-return-a-signal)

@ReactiveCocoa/reactivecocoa This one probably requires more group review than the rest of the guidelines, since this is adding a convention where there wasn't really one before. Lemme know if anything seems unpalatable here.