Add CONTRIBUTING.md

Author: Liuchuan Yu

CONTRIBUTING.md

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+# Contributing to XRoboToolkit-Unity-Client
+
+Thank you for your interest in contributing to XRoboToolkit-Unity-Client! This document provides guidelines and instructions for contributors.
+
+## Prerequisites
+
+### Required Tools
+
+- Unity
+	- 2022.3.16f1 for starting from source
+	- Other versions for starting from UnityPackage
+
+### Recommended Knowledge
+
+Be familiar with Unity development.
+
+## Development Setup
+
+### Start from Source
+
+- git clone https://github.com/XR-Robotics/XRoboToolkit-Unity-Client.git
+- Open with Unity
+- Open Main Scene
+
+### Start from UnityPackage
+
+- Create a new project
+- Download `XRoboToolkit.unitypackage` from the release page
+- Import `XRoboToolkit.unitypackage`
+- Open Main Scene
+
+## License
+
+See the [LICENSE](LICENSE) file for details.
+
+## Licenses and Attributions