add docs link to readme

Author: FoamyGuy

README.rst

@@ -62,6 +62,11 @@ To install in a virtual environment in your current project:
     source .env/bin/activate
     pip3 install adafruit-circuitpython-featherwing
 
+Documentation
+=============
+
+API documentation for this library can be found on `Read the Docs <https://circuitpython.readthedocs.io/projects/featherwing/en/latest/>`_.
+
 Contributing
 ============