SW-14962 - Add unserialize fix to UPGRADE.md

Author: bcremer

UPGRADE.md

@@ -3,9 +3,10 @@ In this document you will find a changelog of the important changes related to t
 
 ## 5.1.6
 * The interface `Enlight_Components_Cron_Adapter` in `engine/Library/Enlight/Components/Cron/Adapter.php` got a new method `getJobByAction`. For default implementation see `engine/Library/Enlight/Components/Cron/Adapter/DBAL.php`.
+* Fix a unserialize regression with PHP 5.6.21 and PHP 7.0.6.
 
 ## 5.1.5
-* The smarty variable `sCategoryInfo` in Listing and Blog controllers is now deprecated and will be removed soon. Use `sCategoryContent` instead, it's a drop in replacement. 
+* The smarty variable `sCategoryInfo` in Listing and Blog controllers is now deprecated and will be removed soon. Use `sCategoryContent` instead, it's a drop in replacement.
 
 ## 5.1.4
 * Customer logout will now regenerate the session id and clear the customers basket.