example to call tud_init() after freeRTOS kernel is started

add note for usb init when using with RTOS

src/tusb.h

@@ -109,6 +109,8 @@
  *  @{ */
 
 // Initialize device/host 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 tusb_init(void);
 
 // Check if stack is initialized