Add CITATION.cff

CITATION.cff

+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+  - family-names: Bott
+    given-names: Judith
+    orcid: https://orcid.org/0000-0002-2018-4754
+  - family-names: Benoit
+    given-names: Laureen
+    orcid: https://orcid.org/0000-0002-7665-0240
+  - family-names: Koltzer
+    given-names: Nora
+    orcid: https://orcid.org/0000-0002-4720-7645
+  - family-names: Anikiev
+    given-names: Denis
+    orcid: https://orcid.org/0000-0002-4729-2659
+title: "Py4HIP: Python tool for Heat-In-Place calculations"
+version: 1.0
+doi: 10.5880/GFZ.4.5.2022.001
+date-released: 2022-04-08