Fix issue #4756: [Documentation] When GITHUB_TOKEN is provided automatically through the UI (#4757)

Co-authored-by: Graham Neubig <neubig@gmail.com>
Co-authored-by: sp.wack <83104063+amanape@users.noreply.github.com>

docs/modules/usage/how-to/gui-mode.md

@@ -19,6 +19,15 @@ OpenHands provides a user-friendly Graphical User Interface (GUI) mode for inter
 3. Enter the corresponding `API Key` for your chosen provider.
 4. Click "Save" to apply the settings.
 
+### GitHub Token Setup
+
+OpenHands automatically exports a `GITHUB_TOKEN` to the shell environment if it is available. This can happen in two ways:
+
+1. Locally (OSS): The user directly inputs their GitHub token.
+2. Online (SaaS): The token is obtained through GitHub OAuth authentication.
+
+When you reach the `/app` route, the app checks if a token is present. If it finds one, it sets it in the environment for the agent to use.
+
 ### Advanced Settings
 
 1. Toggle `Advanced Options` to access additional settings.
@@ -49,3 +58,4 @@ The main interface consists of several key components:
 3. Use one of the recommended models, as described in the [LLMs section](usage/llms/llms.md).
 
 Remember, the GUI mode of OpenHands is designed to make your interaction with the AI assistant as smooth and intuitive as possible. Don't hesitate to explore its features to maximize your productivity.
+