Added README.md

README.md

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+## Build & Run
+
+```bash
+docker compose build
+docker compose up
+```
+
+## Topic
+
+- `/ultrasonic/raw_data` (`std_msgs/UInt8MultiArray`)
+
+## Parameters
+
+| Parameter | Default | Description |
+|-----------|---------|-------------|
+| `serial_port` | `/dev/ttyUSB0` | Serial port path |
+| `baudrate` | `38400` | Baud rate |
+
+## Custom Parameters
+
+```bash
+ros2 run ultrasonic_sensor ultrasonic_node --ros-args -p serial_port:=/dev/ttyACM0
+```
+