Introduce Citation.cff and mention black code style in Contributing.md.

(cherry picked from commit 108d0394) Cherry pick citation.cff and CONTRIBUTION.md changes from !49 (closed)
Closes #102 (closed)

CITATION.cff

+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+type: software
+title: plotID
+authors:
+  - family-names: Mayr
+    given-names: Hannes
+    email: hannes.mayr@stud.tu-darmstadt.de
+    affiliation: Institut für Fluidsystemtechnik - TU Darmstadt
+  - family-names: Hock
+    given-names: Martin
+    email: Martin.Hock@fst.tu-darmstadt.de
+    affiliation: Institut für Fluidsystemtechnik - TU Darmstadt
+    orcid: https://orcid.org/0000-0001-9917-3152
+  - family-names: Richter
+    given-names: Manuela
+    email: Manuela.Richter@fst.tu-darmstadt.de
+    affiliation: Institut für Fluidsystemtechnik - TU Darmstadt
+