docs: add NPM gRPC fix for external HTTPS exposure

Document the fix for 'server closed the stream without sending trailers'
gRPC error when exposing NetBird via Nginx Proxy Manager:
- NPM proxy_host advanced_config with grpc_pass for Signal/Management
- Caddy h2c listener enabled via 'protocols h1 h2 h2c'

README.md

@@ -15,6 +15,7 @@
 |------|------|
 | [QUICKSTART.md](./QUICKSTART.md) | 快速部署步驟（適合複製貼上執行） |
 | [netbird-selfhosted-setup.md](./netbird-selfhosted-setup.md) | 完整安裝指南（含原理說明） |
+| [npm-grpc-fix.md](./npm-grpc-fix.md) | 透過 Nginx Proxy Manager 對外公開時的 gRPC 修復 |
 | [SUMMARY.md](./SUMMARY.md) | 部署摘要與疑難排解記錄 |
 
 ## 🚀 快速開始