Added width & height usage examples

Author: mhanoglu

README.md

@@ -44,6 +44,10 @@ $grid->picture()->gallery();
 //zoom effect
 $grid->picture()->lightbox(['zooming' => true]);
 $grid->picture()->gallery(['zooming' => true]);
+
+//width & height properties
+$grid->picture()->lightbox(['width' => 50, 'height' => 50]);
+$grid->picture()->gallery(['width' => 50, 'height' => 50]);
 ```
 
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