chore: add contribution guide

CONTRIBUTING.md

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+# How to Contribute
+
+The contribution guideline is derived from the SlimPHP, Istanbul Code Coverage and Ruby contribution guideline
+
+## Contributor Behavior
+
+Be kind to one another. We're striving to make Knossos an inclusive environment that's great for first time open-source contributors.
+
+tldr; we value constructive community interaction, over technical acumen.
+
+## Pull Requests
+
+Knossos use the [GitHub flow](https://guides.github.com/introduction/flow/) as main versioning workflow
+
+1. Fork the Knossos repository
+2. Create a new branch for each feature, fix or improvement
+3. Send a pull request from each feature branch to the **main** branch
+
+It is very important to separate new features or improvements into separate feature branches, and to send a pull request for each branch.
+
+This allow us to review and pull in new features or improvements individually.
+
+## Style Guide
+
+All pull requests SHOULD adhere to the [Conventional Commits specification](https://conventionalcommits.org/)
+
+## License
+
+You must agree that your patch will be licensed under the repository Apache License, and when we change the license we will assume that you agreed with the change unless you object to the changes in 30 days time.