docs: how-to-build section

README.md

@@ -1,3 +1,27 @@
 # ChatGPT-API Demo
 
-A demo repo based on [OpenAI GPT-3.5 Turbo API](https://platform.openai.com/docs/guides/chat).
+A demo repo based on [OpenAI GPT-3.5 Turbo API](https://platform.openai.com/docs/guides/chat).
+
+## How to build
+
+1. Setup & Install dependencies
+
+    > First, you need [Node.js](https://nodejs.org/) (v18+) installed.
+
+    ```shell
+    npm i
+    ```
+
+2. Make a copy of `.env.example`, then rename it to `.env`
+3. Add your [OpenAI API key](https://platform.openai.com/account/api-keys) to `.env`
+    ```
+    OPENAI_API_KEY=sk-xxx...
+    ```
+4. Run the app
+    ```shell
+    npm run dev
+    ```
+
+## License
+
+MIT