Add note about Sylius 1.14 in the UPGRADE file (#465)

| Q               | A
| --------------- | -----
| Branch?         | 1.6
| Bug fix?        | no
| New feature?    | no
| Related tickets |

Author: mpysiak

UPGRADE-1.6.md

@@ -1,5 +1,7 @@
 ### UPGRADE FROM 1.5.X TO 1.6.0
 
+1. Support for Sylius 1.14 has been added, it is now the recommended Sylius version to use with RefundPlugin.
+
 1. Support for Sylius 1.12 has been dropped, upgrade your application to [Sylius 1.13](https://github.com/Sylius/Sylius/blob/1.13/UPGRADE-1.13.md).
    or to [Sylius 1.14](https://github.com/Sylius/Sylius/blob/1.14/UPGRADE-1.14.md).