Create CONTRIBUTING.md

CONTRIBUTING.md

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+# How to Contribute to this Repository
+
+Before contributing to this specific repository, please read the [Contribution Guidelines](https://nrkno.github.io/sofie-core/docs/for-developers/contribution-guidelines) for the Sofie project.
+
+
+## Branches
+This repository uses the following branches:
+
+* **_master_** is our main branch. We consider it stable and it is used in production.
+* The **_releaseXX_** branches are our in-development branches. When a release is ready, we decide to “freeze” that branch and create a new **_releaseXX+1_** branch.
+
+We encourage you to base your contributions on the latest **releaseXX** branch, alternatively the **master** branch or a recently frozen **releaseXX** branch. The [Sofie Releases](https://nrkno.github.io/sofie-core/releases) page collects the status and timeline of the releases.