Add github release tag to readme. (#5627)

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>

README.md

@@ -1,5 +1,6 @@
 # <img src="logo.svg" alt="OpenZeppelin" height="40px">
 
+[![Github Release](https://img.shields.io/github/v/tag/OpenZeppelin/openzeppelin-contracts.svg?filter=v*&sort=semver&label=github)](https://github.com/OpenZeppelin/openzeppelin-contracts/releases/latest)
 [![NPM Package](https://img.shields.io/npm/v/@openzeppelin/contracts.svg)](https://www.npmjs.org/package/@openzeppelin/contracts)
 [![Coverage Status](https://codecov.io/gh/OpenZeppelin/openzeppelin-contracts/graph/badge.svg)](https://codecov.io/gh/OpenZeppelin/openzeppelin-contracts)
 [![GitPOAPs](https://public-api.gitpoap.io/v1/repo/OpenZeppelin/openzeppelin-contracts/badge)](https://www.gitpoap.io/gh/OpenZeppelin/openzeppelin-contracts)