Add testing link to usb.md

Apply changes from PR #1096 to v5.13.

Author: Amanda Butler

docs/porting/target/usb.md

@@ -49,3 +49,5 @@ The Mbed OS HAL provides a set of conformance tests for the HAL USBPhy API. You
 ```
 mbed test -t <toolchain> -m <target> -n mbed-os-tests-usb_device-*
 ```
+
+For **setup instructions**, known issues and more information about USB tests, please see the [README](https://github.com/ARMmbed/mbed-os/blob/master/TESTS/usb_device/README.md).