fix: Add documentation on mcp run doesn't support low-level server

Author: john0312

README.md

@@ -346,6 +346,8 @@ python server.py
 mcp run server.py
 ```
 
+Note that `mcp run` or `mcp dev` only supports server using FastMCP and not the low-level server variant.
+
 ## Examples
 
 ### Echo Server
@@ -516,6 +518,8 @@ if __name__ == "__main__":
     asyncio.run(run())
 ```
 
+Caution: The `mcp run` and `mcp dev` tool doesn't support low-level server.
+
 ### Writing MCP Clients
 
 The SDK provides a high-level client interface for connecting to MCP servers: