re-add contributing.md to repo

it is missing in the new branch

CONTRIBUTING.md

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+# Zen Browser Website
+
+Thank you for wanting to contribute to Zen Browser Website. We are thrilled to welcome you to our community. Before you start, please read this document to understand how you can contribute to this project.
+
+## How to Contribute
+
+1. Fork the Zen Browser Website repository.
+2. Clone the Zen Browser Website repository to your local machine.
+
+```bash
+git clone git@github.com:<YOUR_GITHUB_USERNAME>/www.git # SSH
+git clone https://github.com/<YOUR_GITHUB_USERNAME>/www.git # HTTPS
+gh repo clone <YOUR_GITHUB_USERNAME>/www # GitHub CLI
+```
+
+3. Change directory to the cloned repository.
+
+```bash
+cd www
+```
+
+4. Create a branch for your contribution.
+
+```bash
+git checkout -b <BRANCH_NAME>
+```
+
+5. Start the development environment.
+
+```bash
+npm install # or your favorite package manager
+npm run dev
+```
+
+6. Make your changes.
+
+7. Once you're done, commit your changes.
+
+```bash
+git add .
+git commit -m "Your commit message"
+```
+
+> [!NOTE]
+> Please follow the commit guidelines described below.
+
+## Commit Guidelines
+
+This project follows the [Conventional Commits][] specification.
+
+Commits must be signed. You can learn more about [Commit Signing][] here.
+
+### Commit Message Guidelines
+
+- Commit messages must include a "type" as described in Conventional Commits
+- Commit messages **must** start with a capital letter
+- Commit messages **must not** end with a period `.`
+- Commit messages **must** be in English _sorry for the constraint_
+
+[Conventional Commits]: https://www.conventionalcommits.org/
+[Commit Signing]: https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits