UPGRADE.md for 'mongodb' extension added

extensions/mongodb/UPGRADE.md

+Upgrading Instructions for Yii Framework v2
+===========================================
+
+!!!IMPORTANT!!!
+
+The following upgrading instructions are cumulative. That is,
+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 from Yii 2.0.0
+----------------------
+
+* MongoDB PHP extension min version raised up to 1.4.0. You should upgrade your environment in case you are
+  using older version.
+