Update UPGRADE.md

framework/UPGRADE.md

@@ -8,6 +8,16 @@ 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
+----------------------
+
+* Upgraded Twitter Bootstrap to version 3.3.x. If you need to use an older version (i.e. stick with 3.2.x) you can
+  specify that in your `composer.json` by adding the following line in the `require` section:
+  
+  ```json
+  "bower-asset/bootstrap": "3.2.*",
+  ```
+
 Upgrade from Yii 2.0 RC
 -----------------------