Add README for electron demos for indexing (#1274)

This PR adds a comprehensive README file for the electron demos directory to improve discoverability and provide structured documentation for developers exploring rclnodejs integration with Electron applications.

### Key Changes
- Creates a new README.md file with detailed descriptions of available Electron demos
- Organizes demos by complexity level and functionality categories
- Provides development tips and technology stack information

Fix: #1273

Author: minggangw

electron_demo/README.md

@@ -0,0 +1,70 @@
+# Electron Demos
+
+This directory contains interactive Electron applications that demonstrate various rclnodejs features with rich graphical interfaces. Each demo showcases how to integrate ROS2 functionality into desktop applications using web technologies.
+
+## 📱 Available Demos
+
+### 🔤 Basic Communication
+
+- **[topics/](./topics/)** - Minimal publisher/subscriber demo
+  - Basic ROS2 topic communication with simple UI
+  - Publishes and subscribes to string messages
+  - Ideal starting point for learning rclnodejs with Electron
+
+### 🚗 Robot Control & Visualization
+
+- **[car/](./car/)** - Interactive car control with joystick
+
+  - Virtual joystick for velocity command publishing
+  - Real-time car visualization responding to commands
+  - Emergency stop functionality and directional controls
+
+- **[manipulator/](./manipulator/)** - Two-joint robotic arm simulation
+  - 3D robotic arm visualization using Three.js
+  - Interactive joint controls with sliders
+  - Automatic animation patterns and live feedback
+  - ROS2 integration via `sensor_msgs/msg/JointState`
+
+### 🌍 Advanced Robotics Applications
+
+- **[turtle_tf2/](./turtle_tf2/)** - TF2 transforms and turtle simulation
+  - Complete TF2 coordinate frame system implementation
+  - Integration with turtlesim for turtle tracking
+  - 3D WebGL visualization of coordinate transformations
+  - Intelligent turtle following behavior
+
+## Complexity Levels
+
+| Demo            | Complexity        | Features                   | Best For              |
+| --------------- | ----------------- | -------------------------- | --------------------- |
+| **topics**      | ⭐ Beginner       | Basic pub/sub              | Learning fundamentals |
+| **car**         | ⭐⭐ Intermediate | Control UI, visualization  | Interactive control   |
+| **manipulator** | ⭐⭐⭐ Advanced   | 3D graphics, joint control | Robotics simulation   |
+| **turtle_tf2**  | ⭐⭐⭐⭐ Expert   | TF2 system, complex logic  | Transform management  |
+
+## 🛠️ Development Tips
+
+### ROS2 Integration
+
+- All demos use rclnodejs in the renderer process
+- Node lifecycle management follows Electron best practices
+- Proper cleanup on application exit
+
+### UI Technologies
+
+- **Electron**: Desktop application framework
+- **Three.js**: 3D graphics and visualization
+- **Modern CSS**: Responsive and accessible interfaces
+- **Web APIs**: Canvas, WebGL, and DOM manipulation
+
+### Performance Considerations
+
+- Efficient ROS2 message handling
+- Optimized rendering loops
+- Memory management for long-running applications
+
+## 📚 Related Resources
+
+- [rclnodejs Tutorials](../tutorials/) - Core concepts and API usage
+- [ROS2 Documentation](https://docs.ros.org/) - Official ROS2 reference
+- [Electron Documentation](https://www.electronjs.org/docs) - Electron framework guide