Document the usbd_ep_write(dev, ep, 0, 0) trick

[aitap]

Hello and thank you for the wonderfully compact library! After STM32_USB_Device_Library, it's a pleasure to use.

It takes a while to figure out how to use the code properly (took me a few evenings to find out when the usbd_evt_eprx events should actually arrive for my endpoint), so I would like to introduce some more documentation. I can expand this pull request with more information if that is desirable. Please let me know what you think.