Tests: USB: Add a README for Linux users

Explain how to overcome xHCI limitations in kernels prior to v4.17 and
successfully run USB tests.

Author: Filip Jagodzinski

TESTS/usb_device/basic/README.md

@@ -0,0 +1,15 @@
+# Testing the USB device data toggle reset with a Linux host
+
+When you run the `tests-usb_device-basic` test suite on a Linux machine, please make
+sure that at least one of the following prerequisites are met:
+* using the Linux kernel ***4.17* or newer**,
+* using the ***eHCI*** USB driver instead of *xHCI*.
+
+Implementations of the *xHCI* driver prior to version 4.17 of the Linux kernel did
+not have the functionality necessary to test `"endpoint test data toggle reset"`.
+Even though the data toggle is correctly reset on the device side, the host side will
+not be synchronized and the test will falsely fail.
+
+Further reading:
+1. [the Linux kernel patch adding missing xHCI behavior](https://github.com/torvalds/linux/commit/f5249461b504d35aa1a40140983b7ec415807d9e),
+1. [LKML discussion explaining the details of this issue](https://lkml.org/lkml/2016/12/15/388).