add docs link to readme

Author: FoamyGuy

README.rst

@@ -55,6 +55,11 @@ To install in a virtual environment in your current project:
     source .env/bin/activate
     pip3 install adafruit-circuitpython-ble-lywsd03mmc
 
+Documentation
+=============
+
+API documentation for this library can be found on `Read the Docs <https://circuitpython.readthedocs.io/projects/ble_lywsd03mmc/en/latest/>`_.
+
 Contributing
 ============