add rt-thread's tinyusb repo address

README.rst

@@ -91,7 +91,7 @@ TinyUSB is completely thread-safe by pushing all Interrupt Service Request (ISR)
 
 - **No OS**
 - **FreeRTOS**
-- `RT-Thread <https://github.com/RT-Thread/rt-thread>`_
+- `RT-Thread <https://github.com/RT-Thread/rt-thread>`_: `repo <https://github.com/RT-Thread-packages/tinyusb>`_
 - **Mynewt** Due to the newt package build system, Mynewt examples are better to be on its `own repo <https://github.com/hathach/mynewt-tinyusb-example>`_
 
 Local Docs