docs: add javadoc for new method

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/demo/TabbedDemo.java

@@ -115,7 +115,13 @@ public void addDemo(Component demo, String label, String sourceCodeUrl) {
     this.addDemo(demo.getClass(), label, sourceCodeUrl);
   }
 
-  public void addDemo(Class<? extends Component> clazz, String label, String sourceCodeUrl){   
+  /** Add a tab with a demo component.
+   *
+   * @param clazz the class of routed demo view component
+   * @param label the demo name (tab label)
+   * @param sourceCodeUrl the url of the demo, <b>null</b> to not show source code section.
+   */
+  public void addDemo(Class<? extends Component> clazz, String label, String sourceCodeUrl) {
     if(!clazz.isAnnotationPresent(Route.class)) {
       throw new IllegalArgumentException(clazz + " must be annotated as Route");
     }