add CONTRIBUTING.md

Author: FynnBe

CONTRIBUTING.md

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+Thank you for considering to contribute to bioimageio.spec!
+
+Find good first issues [here](https://github.com/bioimage-io/spec-bioimage-io/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22).
+Open or join a discussion [here](https://github.com/bioimage-io/spec-bioimage-io/discussions).
+
+More refined instructions on how to contribute will follow.