Added Black formatting badge

Author: evaherrada

README.rst

@@ -12,6 +12,10 @@ CircuitPython_RGBLED
     :target: https://github.com/adafruit/Adafruit_CircuitPython_RGBLED/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 RGB LEDs. Works with native microcontroller pins,
 `Adafruit Blinka <https://github.com/adafruit/Adafruit_Blinka>`_, or the `PCA9685 PWM driver <https://www.adafruit.com/product/815>`_.