Documented the debug option in readme

Author: Dmitry Igoshin

readme.md

@@ -74,6 +74,12 @@ Check [`bower.json` file](https://github.com/sayanee/angularjs-pdf/blob/master/b
         ```html
         <ng-pdf template-url="/partials/viewer.html" page=12></ng-pdf>
         ```
+    - `debug` to enable debugging console output (optional, disabled by default)
+
+        ```html
+        <ng-pdf template-url="/partials/viewer.html" debug="true"></ng-pdf>
+        ```
+
 
 1. include the `canvas` element to display the pdf in the template-url file