Update README to add link to documentation

Author: tekktrik

README.rst

@@ -108,5 +108,7 @@ before contributing to help this project stay welcoming.
 Documentation
 =============
 
+API documentation for this library can be found on `Read the Docs <https://docs.circuitpython.org/projects/neopixel/en/latest/>`_.
+
 For information on building library documentation, please check out
 `this guide <https://learn.adafruit.com/creating-and-sharing-a-circuitpython-library/sharing-our-docs-on-readthedocs#sphinx-5-1>`_.