Annotate and document that cancel method throws CancellationException

README.md

@@ -334,4 +334,6 @@ cancellableFuture.cancel()
 
 Note that the `.cancel` method on `CancelableFuture` is a best effort implementation and
 it also does not handle cleaning up of resources (such as file handles) since further
-computations deriving from `.map`/`.flatMap`/`onComplete` may not execute.
+computations deriving from `.map`/`.flatMap`/`onComplete` may not execute. If a 
+`CancelableFuture` was cancelled this way it will fail with a `CancellationException`
+exception.

src/main/scala/markatta/futiles/CancellableFuture.scala

@@ -9,6 +9,7 @@ trait CancellableFuture[T] extends Future[T] {
 
   /** Attempts to cancel the underlying [[scala.concurrent.Future]]. Note that this is a best effort attempt
     */
+  @throws[CancellationException]
   def cancel(): Unit
 }