Create CONTRIBUTING.md

Author: koppor

CONTRIBUTING.md

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+# Contributing
+
+We welcome contributions to JabRef and encourage you to follow the [GitHub workflow](https://docs.github.com/en/get-started/using-github/github-flow).
+You can also check out the explanation of [Feature Branch Workflow](https://atlassian.com/git/tutorials/comparing-workflows#feature-branch-workflow) for the idea behind this kind of development.
+
+## Getting a task assigned
+
+Comment on the issue you want to work at with `/assign-me`.
+GitHub will then automatically assign you.
+
+<!-- markdownlint-disable-next-line MD026 -->
+## Give JabRef a Star!
+
+> [!IMPORTANT]
+> JabRef is completely free and used by students and researchers all over the world.
+> It is actively developed and maintained primarily by volunteers in their free time.
+> Keep them motivated by giving the project a GitHub star:
+> Simply navigate to <https://github.com/jabref/jabref/> and click on the Star button!