Added Black formatting badge

Author: evaherrada

README.rst

@@ -14,6 +14,10 @@ Introduction
     :target: https://github.com/adafruit/Adafruit_CircuitPython_MAX7219/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 driver for the MAX7219 LED matrix driver chip.
 
 See `here <https://github.com/adafruit/micropython-adafruit-max7219>`_ for the equivalent MicroPython driver.