Added Black formatting badge

Author: evaherrada

README.rst

@@ -13,6 +13,10 @@ Introduction
     :target: https://github.com/adafruit/Adafruit_CircuitPython_JWT/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
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 JSON Web Token (JWT) Authentication module for CircuitPython. JSON Web Tokens are an open, industry standard
 `RFC 7519 <https://tools.ietf.org/html/rfc7519>`_ method for representing claims securely between two parties.