add CITATION.cff for scientific metadata

CITATION.cff

+# This CITATION.cff file was generated with cffinit.
+# Visit https://bit.ly/cffinit to generate yours today!
+
+cff-version: 1.2.0
+title: toarstats
+message: >-
+  If you use this software, please cite it using the
+  metadata from this file.
+type: software
+authors:
+  - given-names: Niklas
+    family-names: Selke
+    email: n.selke@fz-juelich.de
+    affiliation: Forschungszentrum Jülich
+    orcid: 'https://orcid.org/0000-0002-9954-2250'
+repository-code: >-
+  https://gitlab.jsc.fz-juelich.de/esde/toar-public/toarstats
+license: MIT
\ No newline at end of file