example to call tud_init() after freeRTOS kernel is started

add note for usb init when using with RTOS

src/host/usbh.h

@@ -91,6 +91,9 @@ TU_ATTR_WEAK void tuh_umount_cb(uint8_t dev_addr);
 //--------------------------------------------------------------------+
 // CLASS-USBH & INTERNAL API
 //--------------------------------------------------------------------+
+
+// 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 usbh_init(void);
 bool usbh_control_xfer (uint8_t dev_addr, tusb_control_request_t* request, uint8_t* data);