docs: add CONTRIBUTING.md file

(cherry picked from commit 68ccf614)

Co-authored-by: Jan Wennrich <jan.wennrich@pcsg.de>

CONTRIBUTING.md

+# Contributing
+
+This package follows the [QUIQQER contribution guidelines](https://dev.quiqqer.com/quiqqer/stabilization/documentation/-/wikis/home).
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