add badge

README.md

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 [![PyPI version](https://img.shields.io/pypi/v/PyPEF?color=blue)](https://pypi.org/project/pypef/)
 [![Python version](https://img.shields.io/pypi/pyversions/PyPEF)](https://www.python.org/downloads/)
 [![Build](https://github.com/niklases/PyPEF/actions/workflows/build.yml/badge.svg)](https://github.com/niklases/PyPEF/actions/?query=workflow:build)
+[![PyPI Downloads](https://static.pepy.tech/badge/pypef)](https://pepy.tech/projects/pypef)
 
 a framework written in Python 3 for performing sequence-based machine learning-assisted protein engineering to predict a protein's fitness from its sequence using different forms of sequence encoding:
 - One-hot encoding