docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# 💡 Contributing
+
+Thanks a lot for your interest in contributing to **editorconfig-checker.javascript**! 🎉
+
+## Open Development
+
+All work on **editorconfig-checker.javascript** happens directly on [GitHub](https://github.com/editorconfig-checker/editorconfig-checker.javascript). Both core team members and external contributors send pull requests which go through the same review process.
+
+## Types of contributions
+
+- Reporting a bug.
+- Suggest a new feature idea.
+- Correct spelling errors, improvements or additions to documentation files (README, CONTRIBUTING...).
+- Improve structure/format/performance/refactor/tests of the code.
+
+## Pull Requests
+
+- **Please first discuss** the change you wish to make via [issue](https://github.com/editorconfig-checker/editorconfig-checker.javascript/issues) before making a change. It might avoid a waste of your time.
+
+- Ensure your code respect linting.
+
+- Make sure your **code passes the tests**.
+
+If you're adding new features to **editorconfig-checker.javascript**, please include tests.
+
+## Commits
+
+The commit message guidelines adheres to [Conventional Commits](https://www.conventionalcommits.org/) and [Semantic Versioning](https://semver.org/) for releases.