added citation file

CITATION.cff

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+# YAML 1.2
+# This CITATION.cff file was generated with cffinit.
+# Visit https://bit.ly/cffinit to generate yours today!
+---
+cff-version: 1.2.0
+title: edugame-railway-operation
+message: 'If you use this project, please cite it using these metadata.'
+type: dataset
+authors:
+  - given-names: Martin
+    family-names: Scheidt
+    email: m.scheidt@tu-bs.de
+    orcid: 'https://orcid.org/0000-0002-9384-8945'
+    affiliation: TU Braunschweig
+identifiers:
+  - type: doi
+    value: 10.5281/zenodo.6243948
+    description: 'Collection of archived snapshots of all versions of the library'
+  - type: doi
+    value: 10.5281/zenodo.6244006
+    description: Current version
+repository-code: 'https://repository.ivev.bau.tu-bs.de/IfEV/edugame-railway-operation'
+url: 'https://www.railtoolkit.org/projects/edugame-railway-operation'
+repository: 'https://github.com/railtoolkit/edugame-railway-operation'
+abstract: >-
+  This education game is a physical model made from
+  either paper craft or a wooden railway. It comes
+  with tasks to illustrate block division, routes,
+  timetabling, and infrastructure planning for
+  students to immerse themselves in the problems of
+  rail-guided transport.
+keywords:
+  - Open Educational Resources
+  - railway
+  - railroad
+license: CC-BY-4.0
+version: v0.5
+date-released: '2019-04-01'