Add CONTRIBUTING.md for GitHub compliance

Author: dalito

CONTRIBUTING.md

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+# Contributing to Voc4Cat
+
+Thank you for your interest in contributing to the Voc4Cat vocabulary! This SKOS vocabulary covers catalysis disciplines and welcomes contributions from the community.
+
+## Quick Start
+
+1. **Download** the current vocabulary Excel file: [voc4cat.xlsx](https://nfdi4cat.github.io/voc4cat/dev/voc4cat.xlsx)
+2. **Edit** the Excel file to add/modify concepts
+3. **Request ID range** (for new concepts): [Create an issue](https://github.com/nfdi4cat/voc4cat/issues/new/choose)
+4. **Submit** your Excel file in a pull request:
+   - Place file in `inbox-excel-vocabs/` folder
+   - Keep the filename as `voc4cat.xlsx`
+   - Describe your changes in the PR description
+
+## Important Guidelines
+
+- **Small contributions welcome**: Even single concept additions are valuable
+- **Break up large changes**: Split contributions of 50+ concepts into smaller chunks (~20 changes each)
+- **Use Excel workflow only**: Never edit Turtle (.ttl) files directly
+- **ID ranges required**: Request your ID range before adding new concepts
+
+## Documentation
+
+For detailed guidelines and step-by-step instructions, see:
+
+- **[Current Vocabulary Guidelines](https://nfdi4cat.github.io/voc4cat/docs_usage/guidelines.html)**
+- **[Step-by-step Guide (PDF)](https://zenodo.org/records/13757351)** - Visual walkthrough of the process
+- **[Vocabulary Guidelines V2.0](https://doi.org/10.5281/zenodo.13773255)** - Academic publication on Zenodo
+
+## What Happens Next?
+
+(next after quick start)
+
+1. CI/CD pipeline automatically processes your Excel file
+2. Turtle files are generated and validated
+3. Updated Excel file is created from the processed data
+4. Reviewers will check your contribution
+5. Upon approval, your changes are merged and published
+
+## Questions or Issues?
+
+- **Vocabulary discussions**: [Create an issue](https://github.com/nfdi4cat/voc4cat/issues)
+- **Tooling improvements**: [voc4cat-tool issues](https://github.com/nfdi4cat/voc4cat-tool/issues)
+- **Template feedback**: [voc4cat-template issues](https://github.com/nfdi4cat/voc4cat-template/issues)
+
+## License
+
+All vocabulary contributions are released under CC0-1.0 license (public domain).