Add contributing guidelines to CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing Guidelines
+
+Welcome to the **WordPress Playground** project! 🎉  
+Playground brings WordPress to the browser through WebAssembly — a place where anyone can experiment, test, and build with WordPress instantly. We’re excited to have you here, and everyone is welcome to contribute.
+
+---
+
+## 💡 How Can I Contribute?
+
+WordPress Playground is an open-source project and welcomes all contributors from code to design, and from documentation to triage. If the feature you need is missing, you are more than welcome to start a discussion, open an issue, and even propose a Pull Request to implement it.
+
+Here are a few quickstart guides to get you started:
+
+-   Code contributions – see the [developer section](https://wordpress.github.io/wordpress-playground/contributing/code/).
+-   Documentation – see the [documentation section](https://wordpress.github.io/wordpress-playground/contributing/documentation).
+-   Triage – see the [triage section](https://wordpress.github.io/wordpress-playground/contributing/#triaging-issues).
+-   Contributions to translations – see the [translations section](https://wordpress.github.io/wordpress-playground/contributing/translations).
+-   Reporting bugs – open an [issue](https://github.com/WordPress/wordpress-playground/issues/new/choose) in the repository.
+-   Ideas, designs or anything else – open a [GitHub discussion](https://github.com/WordPress/wordpress-playground/discussions) and let's talk!
+-   Connect with the Playground team at [`#playground` channel on WordPress Slack](https://wordpress.slack.com/archives/C04EWKGDJ0K)
+
+---
+
+## 🧭 Guidelines
+- As with all WordPress projects, we’re committed to a **welcoming, inclusive, and respectful environment** for everyone. Please follow our [Code of Conduct](https://make.wordpress.org/handbook/community-code-of-conduct/).
+- Before submitting a pull request, review the [repository management and pull request best practices](https://wordpress.github.io/wordpress-playground/contributing/code), and follow the WordPress Playground coding and accessibility standards.
+- [WordPress Coding Standards](https://developer.wordpress.org/coding-standards/wordpress-coding-standards/)
+- **Maintain compatibility:** If your changes affect shared functions, modules, or APIs, ensure they don’t break existing Playground environments or integrations.
+- You maintain copyright over your contributions. By submitting a pull request, you agree to release your code under [Playground’s License](/LICENSE.md).
+
+---
+
+Thank you for contributing to WordPress Playground! 💙