chore: add contributing information

Author: sdirix

CONTRIBUTING.md

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+# Contributing to JSON Forms
+
+We enthusiastically welcome participation in the JSON Forms project! Whether you're reporting bugs, proposing features, participating in discussions, or making direct contributions, your involvement is highly valued.
+
+## Ways to Contribute
+
+### Reporting Bugs
+
+If you encounter a bug, please report it via [GitHub issues](https://github.com/eclipsesource/jsonforms/issues/new/choose).
+Be sure to include a detailed description of the bug and steps to reproduce it.
+
+### Feature Proposals
+
+To propose a new feature, you can also use [GitHub issues](https://github.com/eclipsesource/jsonforms/issues/new/choose) or discuss it on our [Community Board](https://jsonforms.discourse.group).
+
+### Discussions and Questions
+
+For general discussions and questions about using JSON Forms, please visit our [Community Board](https://jsonforms.discourse.group).
+
+### Code Contributions
+
+If you'd like to contribute code, please do so via a pull request to one of our repositories.
+Make sure to review the following sections to understand our requirements and recommendations.
+
+## Before Contributing
+
+### Contributor License Agreement (CLA)
+
+All contributors are required to sign a CLA, which will be checked automatically when you open a pull request.
+
+### Contribution Quality
+
+Contributions to the main repository should adhere to the following:
+
+- Pass all automated checks, including test cases, linting, and format checking.
+- Include new unit tests for new code.
+- If appropriate, modify an existing example or add a new one to demonstrate the new feature or improvement.
+
+The better the quality of the contribution, the faster it can be merged.
+
+### Contributing Features
+
+If you have an idea for a feature, we recommend first opening an issue or starting a discussion on the [Community Board](https://jsonforms.discourse.group) to assess whether it aligns with the goals and scope of JSON Forms.
+We aim to keep the core framework lean and clean, focusing on features that provide the most value.
+
+### Contributing Version Upgrades
+
+We strive to always support the latest versions of dependencies, while carefully managing the lower bounds of our (peer) dependency declarations.
+For seed applications, we use the latest dependencies as a testbed to ensure compatibility.
+
+When contributing version upgrades:
+
+- Only raise the lower bound of (peer) dependencies if necessary, or if avoiding such a raise requires substantial effort.
+- Extend the upper bounds to include the latest supported version, which should ideally be the latest available stable version
+- Test the new JSON Forms version in the respective seed application, both with and without updating the corresponding dependencies. Tip: tools like [yalc](https://github.com/wclr/yalc) can be particularly useful for this purpose.
+
+## Repositories
+
+You can contribute to any of the following repositories:
+
+- [JSON Forms Core](https://github.com/eclipsesource/jsonforms)
+- [JSON Forms Website](https://github.com/eclipsesource/jsonforms2-website)
+- [JSON Forms React Seed](https://github.com/eclipsesource/jsonforms-react-seed)
+- [JSON Forms Vue Seed](https://github.com/eclipsesource/jsonforms-vue-seed)
+- [JSON Forms Angular Seed](https://github.com/eclipsesource/jsonforms-angular-seed)
+
+Contributions to our website and documentation are particularly encouraged!