added before/after example

UPGRADE

@@ -9,6 +9,19 @@ information such as the executed SQL statement, and any bound parameters.
 If you want to retrieve the driver specific exception, you can retrieve it by calling the
 ``getPrevious()`` method on DBALException.
 
+Before:
+
+    catch(\PDOException $ex) {
+        // ...
+    }
+
+After:
+
+    catch(\Doctrine\DBAL\DBALException $ex) {
+        $pdoException = $ex->getPrevious();
+        // ...
+    }
+
 ## Doctrine\DBAL\Connection#setCharsetSQL() removed
 
 This method only worked on MySQL and it is considered unsafe on MySQL to use SET NAMES UTF-8 instead