docs(readme): clarify model definition and add usage example for undefined models

- Updated `README.md` and `README_CN.md` to include additional instructions on requesting undefined models using the `openrouter://` format.
- Added example for `moonshotai/kimi-k2:free` usage.

Author: luispater

README.md

@@ -415,7 +415,7 @@ openai-compatibility:
     # api-keys:
     #   - "sk-or-v1-...b780"
     #   - "sk-or-v1-...b781"
-    models: # The models supported by the provider.
+    models: # The models supported by the provider. Or you can use a format such as openrouter://moonshotai/kimi-k2:free to request undefined models
       - name: "moonshotai/kimi-k2:free" # The actual model name.
         alias: "kimi-k2" # The alias used in the API.
 ```

README_CN.md

@@ -428,7 +428,7 @@ openai-compatibility:
     # api-keys:
     #   - "sk-or-v1-...b780"
     #   - "sk-or-v1-...b781"
-    models: # 提供商支持的模型。
+    models: # 提供商支持的模型。或者你可以使用类似 openrouter://moonshotai/kimi-k2:free 这样的格式来请求未在这里定义的模型
       - name: "moonshotai/kimi-k2:free" # 实际的模型名称。
         alias: "kimi-k2" # 在API中使用的别名。
 ```