Added CONTRIBUTING.md for CodeJudge project

Author: naibaf-1

CONTRIBUTING.md

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+# Contributing to CodeJudge
+
+Thank you for your interest in contributing to **CodeJudge**! Your help is appreciated, whether you are reporting bugs, suggesting features, improving documentation, or submitting code.
+
+---
+
+## How to Contribute
+
+### 1. Reporting Bugs
+- Please [open an issue](../../issues) and provide as much detail as possible:
+  - Steps to reproduce the bug
+  - Expected behavior
+  - Actual behavior
+  - Device, OS and the version  of your OS
+  - Screenshots if applicable
+
+### 2. Suggesting Features
+- [Open an issue](../../issues) and use the "Feature request" label.
+- Clearly explain your idea and its benefits.
+- If possible, describe how it could be implemented.
+
+### 3. Improving Documentation
+- Feel free to submit pull requests to improve the README or other documentation.
+- Fixing typos, clarifying instructions, and adding examples are all welcome.
+
+### 4. Code Contributions
+
+#### Getting Started
+- Fork this repository.
+- Clone your fork to your local machine.
+- Create a new branch for your feature or fix.
+
+#### Checklist
+- Write clear, descriptive commit messages.
+- Follow the project’s code style (C as backend & Flutter/Dart as frontend) if possible.
+- Test your changes before submitting.
+- Do **not** commit files generated by your IDE (e.g., `.idea/`).
+
+#### Submitting a Pull Request
+- Push your branch to your fork.
+- Open a pull request against the `main` branch.
+- Describe your changes and reference related issues if applicable.
+
+---
+
+## Code of Conduct
+
+Please be respectful and considerate in your communications and contributions. All interactions must follow the [GitHub Community Guidelines](https://docs.github.com/en/site-policy/github-terms/github-community-guidelines).
+
+---
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the Apache 2.0 License.
+
+## Attribution
+
+If you fork and redistribute CodeJudge, please include the following attribution in your README or documentation:
+
+> Based on CodeJudge by Fabian Roland (naibaf-1)
+
+---
+
+Thank you for helping improve CodeJudge!