docs: update contributing link in readme

Author: consideRatio

README.md

@@ -6,7 +6,7 @@
 [![GitHub](https://img.shields.io/badge/issue_tracking-github-blue.svg)](https://github.com/jupyterhub/binderhub/issues)
 [![Discourse](https://img.shields.io/badge/help_forum-discourse-blue.svg)](https://discourse.jupyter.org/c/binder/binderhub)
 [![Gitter](https://img.shields.io/badge/social_chat-gitter-blue.svg)](https://gitter.im/jupyterhub/binder)
-[![Contribute](https://img.shields.io/badge/I_want_to_contribute!-grey?logo=jupyter)](https://github.com/jupyterhub/binderhub/blob/HEAD/CONTRIBUTING.md)
+[![Contribute](https://img.shields.io/badge/I_want_to_contribute!-grey?logo=jupyter)](https://binderhub.readthedocs.io/en/latest/contribute.html)
 
 ## What is BinderHub?