chore: add CONTRIBUTING.md

CONTRIBUTING.md

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+# How to contribute to Zigfetch
+
+## Did you find a bug?
+
+- Do not open up a GitHub issue if the bug is a security vulnerability, refer to the [security policy](https://github.com/utox39/zigfetch/blob/main/SECURITY.md).
+- Ensure the bug was not already reported by searching on GitHub under [Issues](https://github.com/utox39/zigfetch/issues).
+- If you're unable to find an open issue addressing the problem, open a new one.
+
+## Do you want to make some changes to the code or documentation?
+
+1. Fork the repo.
+2. Create a new branch (because the `main` branch is protected).
+3. Commit and push the changes to the new branch.
+4. Open a pull request.
+
+---
+
+**Thank you for your contribution!**