When we save the order, it would be great if we have order history.
You can check many other shopping cart has this feature, eg: hikashop.
Because we will ship the product once the order completed, so we will need to know exactly time when the order status is completed(not just created)
So for example:
Order place on 01-Jan-2016 00:00:00
Order status changed to 'complete' on 01-Jan-2016 00:10:00
Admin add product #5 05-Jan-2016 00:10:00
...
