Tests: USB: Add a README for Linux users
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.

andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed