Add DOI

CITATION.cff

 title: >-
-  plotID a toolkit for connecting research data and
+  PlotID a toolkit for connecting research data and
   figures
 message: >-
   developed at the Chair of Fluid System, Technische
@@ -15,6 +15,9 @@ authors:
     email: martin.hock@fst.tu-darmstadt.de
     affiliation: 'Chair of Fluid Systems, TU Darmstadt'
     orcid: ' https://orcid.org/0000-0001-9917-3152'
+identifiers:
+  - type: doi
+    value: 10.5281/zenodo.6375058
 cff-version: 1.2.0
 type: software
 repository-code: 'https://git.rwth-aachen.de/plotid/plot_identifier'