added a citation.cff

CITATION.cff

+# see: https://github.com/citation-file-format/citation-file-format/blob/main/schema-guide.md
+
+cff-version: 1.2.0
+title: Takin
+authors:
+  - family-names: WEBER
+    given-names: Tobias
+    orcid: "https://orcid.org/0000-0002-7230-1932"
+    email: tweber@ill.fr
+    affiliation: "Institut Laue-Langevin (ILL), Grenoble, France"
+abstract: "An inelastic neutron scattering software package."
+message: "Please cite using the DOI 10.5281/zenodo.4117437."
+doi: 10.5281/zenodo.4117437
+type: software
+license: GPL-2.0-only
+repository-code: "https://code.ill.fr/scientific-software/takin/core"