Add OAuth authentication documentation

- Document OAuth 2.1 PKCE flow for stdio mode
- Include setup instructions for GitHub OAuth apps
- Document environment variables and CLI flags
- Add usage examples with custom scopes
- Note that OAuth is stdio-only (not for remote server)

Co-authored-by: SamMorrowDrums <4811358+SamMorrowDrums@users.noreply.github.com>

Author: Copilot

README.md

@@ -186,6 +186,42 @@ To keep your GitHub PAT secure and reusable across different MCP hosts:
 
 </details>
 
+### OAuth Authentication (stdio mode)
+
+For stdio mode (local binary execution), you can use OAuth 2.1 with PKCE instead of a Personal Access Token. This provides an interactive browser-based login flow.
+
+#### Prerequisites for OAuth
+
+1. Create a GitHub OAuth App at [https://github.com/settings/developers](https://github.com/settings/developers)
+   - Set the callback URL to `http://localhost` (the actual port will be dynamically assigned)
+   - For public clients, you can use PKCE without a client secret
+
+2. Set your OAuth app credentials:
+   ```bash
+   export GITHUB_OAUTH_CLIENT_ID=your_client_id
+   export GITHUB_OAUTH_CLIENT_SECRET=your_client_secret  # Optional for public clients with PKCE
+   ```
+
+3. Run the server without a PAT:
+   ```bash
+   ./github-mcp-server stdio
+   ```
+
+The server will automatically detect the OAuth configuration and launch your browser for authorization. After you approve, the server will receive the token and start normally.
+
+#### OAuth Configuration Options
+
+- `--oauth-client-id` / `GITHUB_OAUTH_CLIENT_ID` - Your GitHub OAuth app client ID (required for OAuth flow)
+- `--oauth-client-secret` / `GITHUB_OAUTH_CLIENT_SECRET` - Your client secret (optional, PKCE is used)
+- `--oauth-scopes` / `GITHUB_OAUTH_SCOPES` - Comma-separated list of scopes (defaults: `repo,user,gist,notifications,read:org,project`)
+
+Example with custom scopes:
+```bash
+./github-mcp-server stdio --oauth-client-id YOUR_CLIENT_ID --oauth-scopes repo,user
+```
+
+> **Note**: OAuth authentication is only available in stdio mode. For remote server usage, use Personal Access Tokens as described above.
+
 ### GitHub Enterprise Server and Enterprise Cloud with data residency (ghe.com)
 
 The flag `--gh-host` and the environment variable `GITHUB_HOST` can be used to set