docs: add Discord README badge

Author: charleslpan

README.md

@@ -8,6 +8,7 @@
   <a href="https://www.npmjs.com/package/stagereview"><img src="https://img.shields.io/npm/v/stagereview.svg" alt="npm version"></a>
   <a href="https://www.npmjs.com/package/stagereview"><img src="https://img.shields.io/npm/dm/stagereview.svg" alt="npm downloads"></a>
   <a href="https://github.com/ReviewStage/stage-cli/blob/main/LICENSE"><img src="https://img.shields.io/npm/l/stagereview.svg" alt="license"></a>
+  <a href="https://discord.gg/Hs7Eexp3"><img src="https://img.shields.io/badge/Discord-Join-5865F2?logo=discord&logoColor=white" alt="Discord"></a>
 </p>
 
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