docs(README): add discord badge (#223)

Author: sukvvon

README.md

@@ -1,6 +1,6 @@
 ![react-simplikit](./src/public/images/og.png)
 
-# react-simplikit · [![MIT License](https://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/toss/slash/blob/main/LICENSE) [![codecov](https://codecov.io/gh/toss/react-simplikit/graph/badge.svg?token=RHVOZ3J3TU)](https://codecov.io/gh/toss/react-simplikit)
+# react-simplikit · [![MIT License](https://img.shields.io/badge/license-MIT-blue.svg)](https://github.com/toss/slash/blob/main/LICENSE) [![codecov](https://codecov.io/gh/toss/react-simplikit/graph/badge.svg?token=RHVOZ3J3TU)](https://codecov.io/gh/toss/react-simplikit) [![Discord Badge](https://discord.com/api/guilds/1281071127052943361/widget.png?style=shield)](https://discord.gg/vGXbVjP2nY)
 
 English | [Korean](./README-ko_kr.md)