docs: add npm label in readme

Author: huozhi

README.md

@@ -1,5 +1,7 @@
 # Next.js DevTools MCP
 
+[![npm next-devtools-mcp package](https://img.shields.io/npm/v/next-devtools-mcp.svg)](https://npmjs.org/package/next-devtools-mcp)
+
 `next-devtools-mcp` is a Model Context Protocol (MCP) server that provides Next.js development tools and utilities for AI coding assistants like Claude and Cursor.
 
 ## Features