chore: update readme to add discord badge

README.md

@@ -2,6 +2,7 @@
 
 [![CI](https://github.com/sigoden/aichat/actions/workflows/ci.yaml/badge.svg)](https://github.com/sigoden/aichat/actions/workflows/ci.yaml)
 [![Crates](https://img.shields.io/crates/v/aichat.svg)](https://crates.io/crates/aichat)
+[![Discord](https://img.shields.io/discord/1226737085453701222?label=Discord)](https://discord.gg/NYmfN6CA)
 
 All-in-one CLI tool for 10+ AI platforms, including OpenAI, Gemini, Claude, Mistral, Ollama, VertexAI, Ernie, Qianwen, Moonshot...