Added upgrade step for Yii 2.0.17 about yii\web\DbSession (#17228) [s…

…kip ci]

#17188 might brake existing projects when the yii\web\DbSession class is extended.

framework/UPGRADE.md

@@ -51,6 +51,12 @@ if you want to upgrade from version A to version C and there is
 version B between A and C, you need to follow the instructions
 for both A and B.
 
+Upgrade from Yii 2.0.16
+-----------------------
+
+* In case you have extended the `yii\web\DbSession` class you should check if your 
+  custom implementation is compatible with the new `yii\web\DbSession::$fields` attribute.
+  Especially when overriding the `yii\web\DbSession::writeSession($id, $data)` function.
 
 Upgrade from Yii 2.0.15
 -----------------------