feat(readme): add citation
[![Codacy Badge](https://api.codacy.com/project/badge/Grade/eada52ca726e4919923e213b81ee6420)](https://app.codacy.com/gh/odegym/neurodiffeq?utm_source=github.com&utm_medium=referral&utm_content=odegym/neurodiffeq&utm_campaign=Badge_Grade_Settings)
![PyPI](https://img.shields.io/pypi/v/neurodiffeq?color=blueviolet&label=PyPI&logoColor=blueviolet) ![GitHub issues](https://img.shields.io/github/issues/NeuroDiffGym/neurodiffeq?color=green) [![Build Status](https://travis-ci.org/NeuroDiffGym/neurodiffeq.svg?branch=master)](https://travis-ci.org/NeuroDiffGym/neurodiffeq) [![codecov](https://codecov.io/gh/NeuroDiffGym/neurodiffeq/branch/master/graph/badge.svg)](https://codecov.io/gh/NeuroDiffGym/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)
+# Citation
+
+```
+@article{chen2020neurodiffeq,
+ title={NeuroDiffEq: A Python package for solving differential equations with neural networks},
+ author={Chen, Feiyu and Sondak, David and Protopapas, Pavlos and Mattheakis, Marios and Liu, Shuheng and Agarwal, Devansh and Di Giovanni, Marco},
+ journal={Journal of Open Source Software},
+ volume={5},
+ number={46},
+ pages={1931},
+ year={2020}
+}
+```
+
+------
+
+:mortar_board: **Already familiar with neurodiffeq? ** :point_down: **[Jump to FAQs](#faq).**
+
+------
+
# Introduction
`neurodiffeq` is a package for solving differential equations with neural networks. Differential equations are equations that relate some function with its derivatives. They emerge in various scientific and engineering domains. Traditionally these problems can be solved by numerical methods (e.g. finite difference, finite element). While these methods are effective and adequate, their expressibility is limited by their function representation. It would be interesting if we can compute solutions for differential equations that are continuous and differentiable.
