Add Contributing Guidelines

Guidelines were proudly stolen from the frontend repo and modified to better fit this one

CONTRIBUTING.md

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+# Contributing
+Aka. Developer Guidelines
+
+## Intro
+
+We're currently _very_ early in the development stages of the Sublinks federation service. Therefore there aren't too many guidelines yet. However, as you can see below there are some that we would appreciate you to consider before and while you contribute to this project.
+
+As with most things you'll read about in this document, even these guidelines are up for discussion. If you believe anything should be changed please feel free to create a pull request and let us know what and why. Same thing applies if you want to add or stricten/loosen a guideline.
+
+## Tests
+
+Support for tests has been set up. Please submit unit and/or end-to-end tests with all PRs. 100% coverage (at least with unit tests) is a falacy. Ideally we want to strive for "good enough" coverage instead.
+
+## Pull Requests
+
+The pull request flow in this project isn't anything special. We require a pull request to be created before anything is merged into `main`. At least one person must approve the pull request.