Add custom collection view layout example project

AsyncDisplayKit/Details/ASCollectionViewFlowLayoutInspector.h

@@ -15,6 +15,11 @@
 
 @protocol ASCollectionViewLayoutInspecting <NSObject>
 
+/**
+ * Provides the size range needed to measure the collection view's item.
+ */
+- (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForNodeAtIndexPath:(NSIndexPath *)indexPath;
+
 /**
  * Asks the inspector to provide a constrained size range for the given supplementary node.
  */
@@ -30,6 +35,8 @@
  */
 - (NSUInteger)collectionView:(ASCollectionView *)collectionView supplementaryNodesOfKind:(NSString *)kind inSection:(NSUInteger)section;
 
+@optional
+
 /**
  * Allow the inspector to respond to delegate changes.
  *