Move Black badge to CONTRIBUTING.md

Author: stephane-caron

README.md

@@ -4,7 +4,6 @@
 [![Coverage](https://coveralls.io/repos/github/robot-descriptions/robot_descriptions.py/badge.svg?branch=master)](https://coveralls.io/github/robot-descriptions/robot_descriptions.py?branch=master)
 [![PyPI version](https://img.shields.io/pypi/v/robot_descriptions)](https://pypi.org/project/robot_descriptions/)
 [![Contributing](https://img.shields.io/badge/PRs-welcome-green.svg)](https://github.com/robot-descriptions/robot_descriptions.py/tree/master/CONTRIBUTING.md)
-[![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
 
 Import open source robot descriptions as Python modules.