docs: added instructions for obtaining OpenAI API key to README (#421)

* docs: added instructions for obtaining OpenAI API key to README

* docs: move 'Getting an OpenAI API key' before 'Other examples'

Author: vvatanabe

README.md

@@ -52,6 +52,17 @@ func main() {
 
 ```
 
+### Getting an OpenAI API Key:
+
+1. Visit the OpenAI website at [https://platform.openai.com/account/api-keys](https://platform.openai.com/account/api-keys).
+2. If you don't have an account, click on "Sign Up" to create one. If you do, click "Log In".
+3. Once logged in, navigate to your API key management page.
+4. Click on "Create new secret key".
+5. Enter a name for your new key, then click "Create secret key".
+6. Your new API key will be displayed. Use this key to interact with the OpenAI API.
+
+**Note:** Your API key is sensitive information. Do not share it with anyone.
+
 ### Other examples:
 
 <details>