docs: add Discord README badge

Author: charleslpan

README.md

@@ -10,6 +10,10 @@
   <a href="https://github.com/ReviewStage/stage-cli/blob/main/LICENSE"><img src="https://img.shields.io/npm/l/stagereview.svg" alt="license"></a>
 </p>
 
+<p align="center">
+  <a href="https://discord.gg/Hs7Eexp3"><img src="https://img.shields.io/badge/Discord-Join%20us-5865F2?style=for-the-badge&logo=discord&logoColor=white" alt="Join us on Discord"></a>
+</p>
+
 ---
 
 Try the full Stage experience with GitHub integration at [stagereview.app](https://stagereview.app).