add docs link to readme

Author: FoamyGuy

README.rst

@@ -107,6 +107,11 @@ Usage Example
     print('Saving bitmap')
     save_pixels('/sd/test.bmp', bitmap, palette)
 
+Documentation
+=============
+
+API documentation for this library can be found on `Read the Docs <https://circuitpython.readthedocs.io/projects/bitmapsaver/en/latest/>`_.
+
 Contributing
 ============