Add CITATION.cff

CITATION.cff

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+cff-version: 1.2.0
+title: >-
+  epydeck - An EPOCH plasma PIC code input file (deck)
+  reader/writer
+message: 'If you use this software, please cite it using the metadata from this file.'
+type: software
+authors:
+  - family-names: Hill
+    given-names: Peter
+    orcid: 'https://orcid.org/0000-0003-3092-1858'
+    affiliation: University of York
+  - family-names: Adams
+    given-names: Joel L.
+    orcid: 'https://orcid.org/0009-0005-4889-5231'
+    affiliation: University of York
+repository-code: 'https://github.com/PlasmaFAIR/epydeck'
+date-released: '2024-04-22'