add docs link to readme

Author: FoamyGuy

README.rst

@@ -55,6 +55,11 @@ Usage Example
 Please check the `examples folder <https://github.com/adafruit/Adafruit_CircuitPython_MiniMQTT/tree/main/examples>`_
 for usage examples for this library.
 
+Documentation
+=============
+
+API documentation for this library can be found on `Read the Docs <https://circuitpython.readthedocs.io/projects/minimqtt/en/latest/>`_.
+
 Contributing
 ============