example to call tud_init() after freeRTOS kernel is started

add note for usb init when using with RTOS

src/device/usbd.h

@@ -42,6 +42,8 @@
 //--------------------------------------------------------------------+
 
 // Init device stack
+// Note: when using with RTOS, this should be called after scheduler/kernel is started.
+// Otherwise it could cause kernel issue since USB IRQ handler does use RTOS queue API.
 bool tud_init (void);
 
 // Task function should be called in main/rtos loop