doc: Add setting model

Author: jcs090218

README.md

@@ -24,6 +24,7 @@ interactable commands you can use, and those are mostly examples.*
   - [📨 Sending Request](#📨-sending-request)
   - [📢 API functions](#📢-api-functions)
     - [🔍 Parameters](#🔍-parameters)
+- [🖥 Setting Model](#🖥-setting-model)
 - [🔗 References](#🔗-references)
 - [Contribute](#contribute)
 
@@ -115,6 +116,18 @@ The function's parameters are followed in this order:
                    :max-tokens 4069)       ; optional
 ```
 
+## 🖥 Setting Model
+
+Every type of request has a default `model`, and we hope this benefits the users
+to not worry about what model to use for their request! However, if you want to
+use other models, you can use the keyword `:model` to replace them!
+
+```elisp
+(openai-completion ...
+                   ... 
+                   :model "text-davinci-003")  ; replace the default model
+```
+
 ## 🔗 References
 
 - [CodeGPT](https://marketplace.visualstudio.com/items?itemName=timkmecl.codegpt3)