Document missing requirement for CM108 on Linux

[mfncooper]

The User Guide clearly documents (in section 9.4.1.5.1, Getting permission to access /dev/hidraw) the addition of the hidraw device to the 'audio' group. However, what is missing is any mention that the user under which direwolf is run must also be a member of the 'audio' group. Without that, direwolf will still throw permissions errors, since it will still not have access to the hidraw device.