Add human-readable citation guide

Author: carstenartur

CITATION.md

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+# Citation
+
+If you use **Sandbox**, please cite the archived software release and the repository.
+
+## Preferred citation
+
+Carsten Hammer. **Sandbox**. 2026. Software. GitHub repository: https://github.com/carstenartur/sandbox
+
+When a Zenodo DOI is minted for a release, prefer the Zenodo citation shown on the release page and keep the GitHub repository URL as the code source.
+
+## BibTeX template
+
+```bibtex
+@software{hammer_sandbox_2026,
+  author       = {Hammer, Carsten},
+  title        = {Sandbox},
+  publisher    = {GitHub and Zenodo},
+  url          = {https://github.com/carstenartur/sandbox},
+  license      = {EPL-2.0}
+}
+```
+
+## Author identifier
+
+Carsten Hammer's ORCID iD is https://orcid.org/0009-0005-1047-6381.
+
+## What to cite
+
+- Cite the Zenodo DOI for stable archival reference once available.
+- Cite the GitHub repository when referring to the active development version.
+- Cite specific commits or release tags when reproducibility depends on the exact implementation state.