docs: 添加贡献指南

新增 CONTRIBUTING.md（中英双语），规范 PR 提交流程：
- AI 生成代码需本地验证
- 演示截图需打码
- 禁止大量 JS 注入
- 一个 PR 只做一件事

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README_EN.md

@@ -14,6 +14,8 @@ MCP for RedNote (Xiaohongshu) platform.
 
 - My blog article: [haha.ai/xiaohongshu-mcp](https://www.haha.ai/xiaohongshu-mcp)
 
+> **📌 Please read before submitting a PR: [Contributing Guide](./CONTRIBUTING.md)**
+
 **If you encounter any issues, be sure to check [Common Issues and Solutions](https://github.com/xpzouying/xiaohongshu-mcp/issues/56) first.**
 
 After checking the **Common Issues** list, if you still can't resolve your deployment problems, we strongly recommend using another tool I've created: [xpzouying/x-mcp](https://github.com/xpzouying/x-mcp). This tool doesn't require deployment - you only need a browser extension to drive your MCP, making it more user-friendly for non-technical users.