Add StackAggregator doc (#3745)

Author: M-i-k-e-l

docs/getting-started/v8.md

@@ -69,3 +69,7 @@ Removed (use `PanView` instead)
 ### PanResponderView
 Removed (use `PanView` instead)
 
+### StackAggregator
+Use the `backgroundColor` prop instead of `contentContainerStyle={{backgroundColor}}`
+Fix card being transparent on Android
+`onCollapseChanged` will now be called after the animation has ended (as was intended)