Added an example to help out users to set variables

Shows users how to specify view variables by using (extended) method from ZF2's ViewModel.

README.md

@@ -54,11 +54,16 @@ class ReportController extends AbstractActionController
         $pdf->setOption('filename', 'monthly-report'), // Triggers PDF download, automatically appends ".pdf"
         $pdf->setOption('paperSize', 'a4'); // Defaults to "8x11"
         $pdf->setOption('paperOrientation', 'landscape'); // Defaults to "portrait"
-
+
+        // To set view variables
+        $pdf->setVariables(array(
+          'message' => 'Hello'
+        ));
+
         return $pdf;
     }
 }
 ```
 
 ## To-do
-  - Add command line support.
+  - Add command line support.