Added Black formatting badge

Author: evaherrada

README.rst

@@ -13,6 +13,10 @@ Adafruit CircuitPython Library Bundle
     :target: https://travis-ci.com/adafruit/Adafruit_CircuitPython_Bundle
     :alt: Build Status
 
+.. image:: https://img.shields.io/badge/code%20style-black-000000.svg
+    :target: https://github.com/psf/black
+    :alt: Code Style: Black
+
 This repo bundles a bunch of useful CircuitPython libraries into an easy to
 download zip file. CircuitPython boards can ship with the contents of the zip to
 make it easy to provide a lot of libraries by default.