I would also like to know the answer ! I'll try to do some digging to find out 😉

Edit:

Ok so apparently Odoo added an option to enable the customer screen in the Point of Sale config. In order to make this work you need to go to the config page. In the PosBox / Hardware Proxy section, you need to check the Posbox which will make appear a few other option such as ip address of the posbox, cash drawer,  etc. And at the bottom of that list, there is the option to activate the Customer Display. 

Now as far as I understand it, on your PosBox hardware (assuming you have a PosBox hardware setup, otherwise you can always check this out: https://www.odoo.com/documentation/user/11.0/point_of_sale/overview/setup.html), you need to make sure that the module called hw_screen is enabled. That's what I got so far. Now I'll try to figure out how to make the actual screen work with the PosBox...

Edit2: Once you have enabled the screen option, on your posbox, go to the address http://127.0.0.1:8069/point_of_sale/display. This is the address of the customer display. Then start a new POS new session. You will need press on the screen button in the POS to enable the display. And now, every time you do something on your cart, the customer display will update !