Add readthedocs badge (#68)

Add readthedocs badge

README.md

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 # pyiron_contrib
+[![Documentation Status](https://readthedocs.org/projects/pyiron-contrib/badge/?version=latest)](https://pyiron-contrib.readthedocs.io/en/latest/?badge=latest)
 
 This repository is for user developments on top of the pyiron platform, existing as a publicly accessible incubation area for new ideas. As such, code here might be very useful and cutting-edge, but comes without the level of stability or backwards compatibility associated with the main pyiron branch.