update CITATIONS.cff

Author: ufechner7

CITATION.cff

@@ -7,6 +7,10 @@ authors:
   - family-names: "Poland"
     given-names: "Jelle Agatho Wilhelm"
     orcid: "https://orcid.org/0000-0003-3164-5648"
+  - family-names: "van de Lint"
+    given-names: "Bart"
+  - family-names: "Fechner"
+    given-names: "Uwe"
 title: "VortexStepMethod.jl"
 keywords:
   - Airborne Wind Energy, AWE, AWES, Vortex step methods