chore(readme): update PyPI badge
# neurodiffeq
-[![PyPI version](https://badge.fury.io/py/neurodiffeq.svg)](https://badge.fury.io/py/neurodiffeq) ![GitHub issues](https://img.shields.io/github/issues/odegym/neurodiffeq?color=green) [![Build Status](https://travis-ci.org/odegym/neurodiffeq.svg?branch=master)](https://travis-ci.org/odegym/neurodiffeq) [![codecov](https://codecov.io/gh/odegym/neurodiffeq/branch/master/graph/badge.svg)](https://codecov.io/gh/odegym/neurodiffeq) [![Documentation Status](https://readthedocs.org/projects/neurodiffeq/badge/?version=latest)](https://neurodiffeq.readthedocs.io/en/latest/?badge=latest) [![DOI](https://joss.theoj.org/papers/10.21105/joss.01931/status.svg)](https://doi.org/10.21105/joss.01931)
+![PyPI](https://img.shields.io/pypi/v/neurodiffeq?color=green&label=PyPI%20%28pip%20install%29&logo=PyPI)![GitHub issues](https://img.shields.io/github/issues/odegym/neurodiffeq?color=green) [![Build Status](https://travis-ci.org/odegym/neurodiffeq.svg?branch=master)](https://travis-ci.org/odegym/neurodiffeq) [![codecov](https://codecov.io/gh/odegym/neurodiffeq/branch/master/graph/badge.svg)](https://codecov.io/gh/odegym/neurodiffeq) [![Documentation Status](https://readthedocs.org/projects/neurodiffeq/badge/?version=latest)](https://neurodiffeq.readthedocs.io/en/latest/?badge=latest) [![DOI](https://joss.theoj.org/papers/10.21105/joss.01931/status.svg)](https://doi.org/10.21105/joss.01931)
# Introduction
