Add usage

Author: jcs090218

README.md

@@ -37,6 +37,34 @@ retrieve the key from an auth-source entry under the `:host` key `api.google-gem
 > The two variables `google-gemini-key` is the default values for sending requests!
 > However, you can still overwrite the value by passing the keywords `:key`!
 
+## 🔰 The simplest example
+
+Here is the simplest example that teaches you how to use this library.
+This is a function with a `query` and a callback function.
+
+```elisp
+(google-gemini-content-generate "How are you?"
+                                (lambda (data)
+                                  (message "%s" data)))
+```
+
+## 📨 Sending Request
+
+All arguments are exposed in the argument list, so you can send any request in any way you want.
+
+For example, the request function `google-gemini-content-generate` accepts
+argument max-tokens. By seeing Google Gemini's references page:
+
+> Max output tokens: Specifies the maximum number of tokens that can be generated
+> in the response. A token is approximately four characters. 100 tokens correspond
+> to roughly 60-80 words.
+
+```elisp
+(google-gemini-content-generate ...
+                                ...
+                                :max-output-tokens 100)  ; Get roughly 60-80 words!
+```
+
 ## 🔗 References
 
 - [llm](https://github.com/ahyatt/llm)
@@ -53,7 +81,6 @@ clone and make pull requests to this repository. Or you can
 clone the project and establish your own branch of this tool.
 Any methods are welcome!
 
-
 ### 🔬 Development
 
 To run the test locally, you will need the following tools: