Intro section
"results" > "events" in section about delivery
"Signal events are serialized"
"Subscription will always occur on a scheduler"
"Errors are propagated immediately"
"Subscriptions are automatically disposed"
"Disposal cancels work and cleans up resources"
Note that the main thread always has a scheduler
Merge pull request #311 from ReactiveCocoa/signal-contract

RACSignal contract