add pitfall comment to MVMCoreViewControllerAnimatedTransitioning protocol

MVMCore/MVMCore/PresentationHandling/MVMCoreViewControllerAnimatedTransitioning.h

@@ -12,6 +12,10 @@
 
 @required
 
+/** Property that notifies the navigation controller if the transition was cancelled.
+ *
+ * This property must be KVO compliant (i.e. in Swift add '\@objc dynamic')!
+ */
 @property (nonatomic) BOOL interactiveTransitionCanceled;
 
 @end