docs: add contributing guide

CONTRIBUTING.md

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+# How to contribute to GitHub Slug Action
+
+## Did you find a bug
+
+* **Do not open up a GitHub issue if the bug is a security vulnerability**, and instead to refer to our [security policy][1].
+
+* **Ensure the bug was not already reported** by searching on GitHub under [Issues][2].
+
+* If you're unable to find an open issue addressing the problem, [open a 'Bug' issue][3].
+Be sure to include a **title and clear description**, as much relevant information as possible, and a **code sample** or an **executable test case** demonstrating the expected behavior that is not occurring.
+
+## Did you write a patch that fixes a bug
+
+* Open a new GitHub pull request with the patch.
+
+* Ensure the PR description clearly describes the problem and solution.
+Include the relevant issue number if applicable.
+
+## Do you intend to add a new feature or change an existing one
+
+* Suggest your change by [opening a 'Feature request' issue][3] and start writing code.
+
+## Do you have questions about the source code
+
+* [open a issue][3] with your question.
+
+Thanks!
+
+[1]: https://github.com/rlespinasse/github-slug-action/security/policy
+[2]: https://github.com/rlespinasse/github-slug-action/issues
+[3]: https://github.com/rlespinasse/github-slug-action/issues/new