Add warning to prevent changes to benchmark api server (#2858)

vllm/entrypoints/api_server.py

+"""
+NOTE: This API server is used only for demonstrating usage of AsyncEngine and simple performance benchmarks.
+It is not intended for production use. For production use, we recommend using our OpenAI compatible server.
+We are also not going to accept PRs modifying this file, please change `vllm/entrypoints/openai/api_server.py` instead.
+"""
+
 import argparse
 import json
 from typing import AsyncGenerator