Add contributor badge
:target: https://gitter.im/jupyterhub/binder
:alt: Gitter
+.. image:: https://img.shields.io/badge/I_want_to_contribute!-grey?logo=jupyter
+ :target: https://github.com/jupyterhub/binderhub/blob/master/CONTRIBUTING.md
+ :alt: Contribute
+
What is BinderHub?
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