doc (readme): cmd explained

README.md

@@ -48,6 +48,8 @@ pip install pdf2zh
 
 Execute the translation command in the command line to generate the translated document `example-zh.pdf` and the bilingual document `example-dual.pdf` in the current directory. Use Google as the default translation service.
 
+![](./docs/images/cmd.explained.png)
+
 Please refer to [ChatGPT](https://chatgpt.com/share/6734a83d-9d48-800e-8a46-f57ca6e8bcb4) for how to set environment variables.
 
 <h2 id="partial">Full / partial document translation</h3>

README_zh-CN.md

@@ -48,6 +48,8 @@ pip install pdf2zh
 
 在命令行中执行翻译命令，生成译文文档 `example-zh.pdf` 和双语对照文档 `example-dual.pdf`，默认使用 Google 翻译服务
 
+![](./docs/images/cmd.explained.zh.png)
+
 关于设置环境变量的详细说明，请参考 [ChatGPT](https://chatgpt.com/share/6734a83d-9d48-800e-8a46-f57ca6e8bcb4)
 
 <h2 id="partial">全文或部分文档翻译</h3>