adding badge for the readme

Author: Forbu

README.md

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+[![Generic badge](https://img.shields.io/badge/License-MIT-<COLOR>.svg)](https://shields.io/)
+![PyPI version](https://badge.fury.io/py/fourier-neural-operator.svg)
+
+[![Downloads](https://pepy.tech/badge/fourier-neural-operator)](https://pepy.tech/project/fourier-neural-operator)
+[![Downloads](https://pepy.tech/badge/fourier-neural-operator/month)](https://pepy.tech/project/fourier-neural-operator)
+[![Downloads](https://pepy.tech/badge/fourier-neural-operator/week)](https://pepy.tech/project/fourier-neural-operator)
+
+
 # Fourier Neural Operator package
 
 This code come from the original code https://github.com/zongyi-li/fourier_neural_operator