Update UPGRADE.md

Author: unclead

UPGRADE.md

@@ -8,6 +8,10 @@ if you want to upgrade from version A to version C and there is
 version B between A and C, you need to following the instructions
 for both A and B.
 
+Upgrade 2.11.0
+---------------------------
+- Ensure you use yii2 2.0.13 and higher, otherwise you have to use previous version of the widget
+
 Upgrade from 2.2.0 tp 2.3.0
 ---------------------------