readme 添加 DEFAULT_MODEL 参数 (#4915)

* README_CN.md 添加 DEFAULT_MODEL 的说明

更改默认模型, 很久之前就有大佬支持了, 但更多人只会看readme, readme没有的就以为不支持(包括我).

https://github.com/ChatGPTNextWeb/ChatGPT-Next-Web/pull/4545

* Update README.md

Change default model, it has been supported by experts long ago, but more people only read the readme. If it's not in the readme, they assume it's not supported (including me).

https://github.com/ChatGPTNextWeb/ChatGPT-Next-Web/pull/4545

* Update README.md

ch to en

* en2cn

* 保持位置和readme.md一致

README.md

@@ -245,6 +245,10 @@ To control custom models, use `+` to add a custom model, use `-` to hide a model
 
 User `-all` to disable all default models, `+all` to enable all default models.
 
+### `DEFAULT_MODEL` （optional）
+
+Change default model
+
 ### `WHITE_WEBDEV_ENDPOINTS` (optional)
 
 You can use this option if you want to increase the number of webdav service addresses you are allowed to access, as required by the format：