Add black badge

Author: chkoar

README.rst

@@ -27,7 +27,7 @@
 .. |Gitter| image:: https://badges.gitter.im/scikit-learn-contrib/imbalanced-learn.svg
 .. _Gitter: https://gitter.im/scikit-learn-contrib/imbalanced-learn?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge
 
-.. image:: https://img.shields.io/badge/code%20style-black-000000.svg
+.. |Black| image:: https://img.shields.io/badge/code%20style-black-000000.svg
     :target: https://github.com/psf/black