readme update readthedoc

Author: rflamary

README.md

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 # POT: Python Optimal Transport
 
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+[![Documentation Status](https://readthedocs.org/projects/pot/badge/?version=latest)](http://pot.readthedocs.io/en/latest/?badge=latest)
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 This open source Python library provide several solvers for optimization problems related to Optimal Transport for signal, image processing and machine learning.
 
 It provides the following solvers: