Added a 3D-printable case

[https://www.printables.com/model/1766193-xteink-x4-sliding-cover-w-strap-and-lanyard](https://www.printables.com/model/1766193-xteink-x4-sliding-cover-w-strap-and-lanyard)

The file has not my initials ;) I added 1 - lanyard holes 2 - access to USB port and pushing point 3 - slot for an NFC tag - you can use it as a business card 4 - holes and space for an elastic strap

I have made space for adhesive lining. You have to glue the strap in its slot before adding the lining.