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

(cherry picked from commit 108d0394)

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
+

CONTRIBUTING.md

 # Contributing
 Feel free to **report** all **issues** you encounter with plotID in our [issue tracker](https://git.rwth-aachen.de/plotid/plotid_python/-/issues). This will greatly help to improve plotID.  
-Contributing to plotID via **merge requests** is also highly appreciated. Please make sure that your code complies with the [PEP 8 - Style Guide](https://peps.python.org/pep-0008/) before creating a merge request. We try to have the whole code of plotID covered by unittests. So if you contribute new code to plotid please also provide unittests that cover the new functionality. Should you be interested in adding another plot engine to be supported by plotID, have a look at the section [Implement a new plot engine](#add-plot-engine). There are some hints how to proceed in this case.
+Contributing to plotID via **merge requests** is also highly appreciated. Please make sure that your code complies with the [PEP 8 - Style Guide](https://peps.python.org/pep-0008/) before creating a merge request. We enforce the [**black code style**](https://black.readthedocs.io/en/stable/). We try to have the whole code of plotID covered by unittests. So if you contribute new code to plotid please also provide unittests that cover the new functionality. Should you be interested in adding another plot engine to be supported by plotID, have a look at the section [Implement a new plot engine](#add-plot-engine). There are some hints how to proceed in this case.
 
 ## Setup development environment
 Clone the repository and install the dependencies: