Update README.md

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Author: mvousden

README.md

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 # Virtualmicromagnetics
 
-[![Documentation Status](https://readthedocs.org/projects/virtual-micromagnetics/badge/?version=latest)](http://virtual-micromagnetics.readthedocs.org/en/latest/?badge=latest)
+[![Documentation Status](https://readthedocs.org/projects/virtual-micromagnetics/badge/?version=development)](http://virtual-micromagnetics.readthedocs.org/en/development)
 
 ## For users who want to run micromagnetic simulations