docs(README): 添加环境变量配置说明

- 在 README.md 文件中补充了环境变量配置步骤
- 新增 OPENAI_BASE_URL、OPENAI_API_KEY、LLM_MODEL 和 EMBEDDING_MODEL 环境变量的说明
- 为使用 OpenAI API 和自定义模型提供了更详细的配置指南

Author: monk-after-90s

README.md

@@ -498,6 +498,10 @@ pip install fastapi uvicorn pydantic
 2. Set up your environment variables:
 ```bash
 export RAG_DIR="your_index_directory"  # Optional: Defaults to "index_default"
+export OPENAI_BASE_URL="Your OpenAI API base URL"  # Optional: Defaults to "https://api.openai.com/v1"
+export OPENAI_API_KEY="Your OpenAI API key"  # Required
+export LLM_MODEL="Your LLM model" # Optional: Defaults to "gpt-4o-mini" 
+export EMBEDDING_MODEL="Your embedding model" # Optional: Defaults to "text-embedding-3-large" 
 ```
 
 3. Run the API server: