Added missing image to user-guide.md

Author: githubuser0xFFFF

doc/user-guide.md

@@ -170,4 +170,6 @@ area. In order to give the perfect impression, you should disable the flags
 CDockManager::setConfigFlag(CDockManager::DragPreviewIsDynamic, true);
 CDockManager::setConfigFlag(CDockManager::DragPreviewShowsContentPixmap, false);
 CDockManager::setConfigFlag(CDockManager::DragPreviewHasWindowFrame, false);
-```
+```
+
+![DragPreviewIsDynamic true](dynamic_drag_preview.gif)