chore: update readme about dev config

README.md

@@ -65,6 +65,14 @@ You can star or watch this project or follow author to get release notifictions
 
 [![Open in Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/Yidadaa/ChatGPT-Next-Web)
 
+在开始写代码之前，需要在项目根目录新建一个 `.env.local` 文件，里面填入环境变量：
+
+Before development, you must create a new `.env.local` file at project root, and place your api key into it:
+
+```
+OPENAI_API_KEY=<your api key here>
+```
+
 ## 截图 Screenshots
 
 ![设置 Settings](./static/settings.png)