[README] Add GitPOAP Badge to Display Number of Minted GitPOAPs for C…

…ontributors (ethereum#872)

README.md

@@ -2,6 +2,7 @@
 
 [![License: CC BY-SA 4.0](https://img.shields.io/badge/License-CC%20BY--SA%204.0-lightgrey.svg)](https://creativecommons.org/licenses/by-sa/4.0/)
 [![Gitter](https://badges.gitter.im/ethereum/yellowpaper.svg)](https://gitter.im/ethereum/yellowpaper?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![GitPOAP Badge](https://public-api.gitpoap.io/v1/repo/ethereum/yellowpaper/badge)](https://www.gitpoap.io/gh/ethereum/yellowpaper)
 
 The Yellow Paper is a formal definition of the Ethereum protocol, originally by Gavin Wood, currently maintained by Nick Savers and with contributions from many people around the world.