add Zenodo badge (#160)

Author: simeonschaub

README.md

@@ -8,6 +8,7 @@
 [![Latest Docs](https://img.shields.io/badge/docs-latest-blue.svg)](https://juliadiff.org/FiniteDifferences.jl/latest/)
 [![Code Style: Blue](https://img.shields.io/badge/code%20style-blue-4495d1.svg)](https://github.com/invenia/BlueStyle)
 [![ColPrac: Contributor's Guide on Collaborative Practices for Community Packages](https://img.shields.io/badge/ColPrac-Contributor's%20Guide-blueviolet)](https://github.com/SciML/ColPrac)
+[![DOI](https://zenodo.org/badge/117872213.svg)](https://zenodo.org/badge/latestdoi/117872213)
 
 FiniteDifferences.jl estimates derivatives with [finite differences](https://en.wikipedia.org/wiki/Finite_difference).