Add CITATION.cff

CITATION.cff

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+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+  - family-names: Fabisch
+    given-names: Alexander
+    orcid: https://orcid.org/0000-0003-2824-7956
+  - family-names: Behn
+    given-names: Maarten
+title: "distance3d: Distance computation and collision detection in 3D"
+doi: 10.5281/zenodo.6509736
+url: https://github.com/AlexanderFabisch/distance3d
+type: software