Add code style black to the generated README
.. image:: https://img.shields.io/badge/built%20with-Cookiecutter%20Django-ff69b4.svg
:target: https://github.com/pydanny/cookiecutter-django/
:alt: Built with Cookiecutter Django
+.. image:: https://img.shields.io/badge/code%20style-black-000000.svg
+ :target: https://github.com/ambv/black
+ :alt: Black code style
{% if cookiecutter.open_source_license != "Not open source" %}
:License: {{cookiecutter.open_source_license}}
