examples/usb: Add README that points out the alternative usb modules.

If someone starts from this directory then they won't know they exist,
otherwise.
Signed-off-by: Angus Gratton <angus@redyak.com.au>

examples/usb/README.md

+## USBDevice Examples
+
+These are examples of how to use the low-level `machine.USBDevice` class to create custom
+USB device functionality.
+
+Alternatively, there are [usb driver modules in
+micropython-lib](https://github.com/micropython/micropython-lib/tree/master/micropython/usb#readme)
+which provide a simpler interface and more built-in functionality.
+
+In this directory:
+
+* `usb_dfu_device.py` implements the USB Device Firmware Update protocol.
+* `usb_simple_device.py` implements a custom USB protocol, with a matching host
+  program to run on a PC.
+
+See comments at the top of each Python file for more details.