cookie README: add 'code style black' badge

Author: sommersoft

{{ cookiecutter.library_name }}/README.rst

@@ -25,6 +25,10 @@ Introduction
     :target: https://github.com/{{ full_repo_name }}/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
+
 {% if cookiecutter.library_description != "" %}
     {{- cookiecutter.library_description }}
 {% else %}