Update README.md

Add DOI to JOSS article

Author: Flaminietta

README.md

@@ -9,7 +9,7 @@
 ![PyPI](https://img.shields.io/pypi/v/ProMCDA?label=pypi%20package)
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+[![DOI](https://joss.theoj.org/papers/10.21105/joss.06190/status.svg)](https://doi.org/10.21105/joss.06190)
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