added citation file for ZENODO

Author: FlorianKummer

CITATION.cff

@@ -0,0 +1,21 @@
+# This CITATION.cff file was generated with cffinit.
+# Visit https://bit.ly/cffinit to generate yours today!
+
+cff-version: 1.2.0
+title: foam-dg
+message: >-
+  If you use this software, please cite it using the
+  metadata from this file.
+type: software
+authors:
+  - given-names: Dario
+    family-names: Klingenberg
+    orcid: 'https://orcid.org/0000-1234-5678-0000'
+  - given-names: Hrvoje
+    family-names: Jasak
+  - given-names: Holger
+    family-names: Marschall
+  - given-names: Florian
+    family-names: Kummer
+    email: kummer@fdy.tu-darmstadt.de
+    orcid: 'https://orcid.org/0000-0002-2827-7576'