Create CITATION.cff

CITATION.cff

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+cff-version: 1.2.0
+abstract: "Oxigraph is a graph database implementing the SPARQL standard."
+authors:
+  - given-names: Thomas
+    family-names: Pellissier Tanon
+    email: thomas@pellissier-tanon.fr
+    orcid: "https://orcid.org/0000-0002-0620-6486"
+license: "MIT OR Apache-2.0"
+message: "If you use this software, please cite it as below."
+repository-code: "https://github.com/oxigraph/oxigraph"
+title: Oxigraph
+type: software
+url: "https://oxigraph.org"