comment for check images in `kf_decodedImage(scale:)`

Kingfisher/UIImage+Extension.swift

@@ -67,6 +67,7 @@ extension UIImage {
     }
     
     func kf_decodedImage(scale scale: CGFloat) -> UIImage? {
+        // prevent animated image (GIF) lose it's images
         if images != nil {
             return self
         }