docs(CONTRIBUTING.md): Add contributing guidelines

Author: vontanne

.github/CONTRIBUTING.md

@@ -0,0 +1,20 @@
+# Contributing to "JavaScript Tetris"
+
+Thank you for your interest in contributing to the "JavaScript Tetris"! Every contribution is appreciated.
+
+## Quickstart
+
+1. **Report Issues**: If you find a bug or have a suggestion, please open an issue [here](https://github.com/vontanne/javascript-tetris/issues).
+2. **Submit Pull Requests**: Feel free to take on open issues or enhance the project. Fork the repo, make your changes, and submit a pull request.
+
+## Guidelines
+
+- Ensure your code is well-documented and follows the project's style.
+- Include a clear and descriptive commit message.
+- Pull requests should be based on the latest master branch and should be conflict-free.
+
+## Questions?
+
+If you have any questions or need further clarification, feel free to open an issue for discussion.
+
+Thank you for contributing to the "JavaScript Tetris"!