Add the @discardableResult attribute to a Disposable.add overload
Add the @discardableResult attribute to a Disposable.add overload

According to the documentation comment (and correctly so, IMO), it seems like the return value of this `Disposable.add` function overload is discardable.
ReactiveCocoa/ReactiveSwift,ReactiveCocoa/ReactiveSwift,ReactiveCocoa/ReactiveSwift