docs: mention creating issues first before sending large PR with new feature

Author: wzxxing

CONTRIBUTING.md

@@ -4,6 +4,8 @@ Thank you for your interest in contributing to the MCP Proxy for AWS! We welcome
 
 ## Quick Start
 
+**To avoid wasting your time on contributions that is not aligned with our project, we advice you to create an issue first.**
+
 1. **Fork the repository** on GitHub
 2. **Set up your development environment** - see [DEVELOPMENT.md](DEVELOPMENT.md) for detailed setup instructions
 3. **Create a feature branch** from `main`