[unity] added 3.8 addition changelog section to 3.9 changelog section as well.

CHANGELOG.md

@@ -46,6 +46,7 @@
 		by e.g. the LWRP renderer, leading to incorrect draw order (e.g. "A1 B A2" changed to "A1A2 B").
 		You can leave this parameter disabled when everything is drawn correctly to save the additional performance cost.
   * **Additional Timeline features.** SpineAnimationStateClip now provides a `Speed Multiplier`, a start time offset parameter `Clip In`, support for blending successive animations by overlapping tracks. An additional `Use Blend Duration` parameter *(defaults to true)* allows for automatic synchronisation of MixDuration with the current overlap blend duration. An additional Spine preferences parameter `Use Blend Duration` has been added which can be disabled to default to the previous behaviour before this update.
+  * Additional `SpriteMask and RectMask2D` example scene added for demonstration of mask setup and interaction.
 
 * **Changes of default values**