docs: add dataset citation metadata

CITATION.cff

@@ -0,0 +1,20 @@
+cff-version: 1.2.0
+title: "OSM Polygon Wikidata and Wikipedia, V2 dataset"
+message: "If you use this dataset, please cite it using this metadata."
+type: dataset
+authors:
+  - family-names: Flandre
+    given-names: Noé
+    website: https://noeflandre.com
+version: "2.0.0"
+date-released: "2026-08-13"
+abstract: "A multilingual geospatial dataset of OpenStreetMap polygons discovered through Wikidata and Wikipedia tags, with linked Wikipedia and Wikivoyage documents, sections, and Wikidata facts."
+repository-code: https://github.com/NoeFlandre/osm-polygon-wikidata-only
+url: https://huggingface.co/datasets/NoeFlandre/osm-polygon-wikidata-and-wikipedia
+keywords:
+  - OpenStreetMap
+  - Wikidata
+  - Wikipedia
+  - Wikivoyage
+  - geospatial
+  - parquet

README.md

@@ -115,3 +115,7 @@ V2 deduplicates documents by `document_id` and polygon-document links by `(polyg
 ## Reproducibility
 
 Run V2 explicitly with `sync-dir --dataset-version v2`. V1 remains a separate contract and is not modified by V2 processing.
+
+## Citation
+
+If you use this dataset, please cite it. Download the dataset citation metadata from [`CITATION.cff`](CITATION.cff).