Added Black formatting badge

Author: evaherrada

README.rst

@@ -13,6 +13,10 @@ Introduction
     :target: https://github.com/adafruit/Adafruit_CircuitPython_RSA/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
+
 RSA implementation based on `Sybren A. Stüvel's python-rsa <https://github.com/sybrenstuvel/python-rsa>`_ pure-python
 RSA implementation.