add Zenodo badge (#352)

Author: simeonschaub

README.md

@@ -6,6 +6,7 @@
 [![Coverage](https://codecov.io/gh/JuliaDiff/ChainRulesCore.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/JuliaDiff/ChainRulesCore.jl)
 [![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/199721843.svg)](https://zenodo.org/badge/latestdoi/199721843)
 
 **Docs:**
 [![](https://img.shields.io/badge/docs-master-blue.svg)](https://juliadiff.org/ChainRulesCore.jl/dev)