Add license badge (OpenZeppelin#1151)

Author: Olexandr88

README.md

@@ -2,6 +2,7 @@
 
 [![Docs](https://img.shields.io/badge/docs-%F0%9F%93%84-blue)](https://docs.openzeppelin.com/upgrades-plugins)
 [![Coverage Status](https://codecov.io/gh/OpenZeppelin/openzeppelin-upgrades/graph/badge.svg)](https://codecov.io/gh/OpenZeppelin/openzeppelin-upgrades)
+[![License](https://img.shields.io/github/license/OpenZeppelin/openzeppelin-upgrades)](https://github.com/OpenZeppelin/openzeppelin-upgrades/blob/master/LICENSE)
 
 **Integrate upgrades into your existing workflow.** Plugins for [Hardhat](https://hardhat.org/) and [Foundry](https://book.getfoundry.sh/) to deploy and manage upgradeable contracts on Ethereum.