Add JOSS badge
.. image:: https://readthedocs.org/projects/umap-learn/badge/?version=latest
:target: https://umap-learn.readthedocs.io/en/latest/?badge=latest
:alt: Documentation Status
+.. image:: http://joss.theoj.org/papers/10.21105/joss.00861/status.svg
+ :target: https://doi.org/10.21105/joss.00861
+ :alt: JOSS article for this repository
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UMAP
