Added Black formatting badge

Author: evaherrada

README.rst

@@ -13,6 +13,10 @@ Adafruit_CircuitPython_AdafruitIO
     :target: https://github.com/adafruit/Adafruit_CircuitPython_AdafruitIO/actions/
     :alt: Build Status
 
+.. image:: https://img.shields.io/badge/code%20style-black-000000.svg
+    :target: https://github.com/psf/black
+    :alt: Code Style: Black
+
 CircuitPython wrapper library for communicating with `Adafruit IO <http://io.adafruit.com>`_.