Fix CONTRIBUTING.md (#3365)

Added guidelines for contributing via pull requests.

Author: markus-moser

CONTRIBUTING.md

@@ -1 +1,10 @@
+## Contributing Studio Pull Requests
+We gladly accept community pull requests. This is the preferred way to contribute to Pimcore. There are a few necessary steps before we can accept a pull request:
 
+* [Fork us!](https://help.github.com/articles/fork-a-repo/)
+* Select the right branch. `main`(`2026.x`) for features and improvements or latest maintenance branch for bug fixes (`2025.4`)
+* [Send a pull request](https://help.github.com/articles/using-pull-requests/) from your fork’s branch to our repo branch.
+* [Sign the CLA](https://cla-assistant.io/pimcore/pimcore) - see also below.
+* Ensure that Github pipelines pass (linter, frontend build...)
+
+For details please check also the contributing docs of [pimcore/pimcore](https://github.com/pimcore/pimcore/blob/2026.x/CONTRIBUTING.md).