added CITATION file (#9)

CITATION.cff

@@ -0,0 +1,20 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+- family-names: "Gowers"
+  given-names: "Richard J."
+  orcid: "https://orcid.org/0000-0002-3241-1846"
+- family-names: "Alibay"
+  given-names: "Irfan"
+  orcid: "https://orcid.org/0000-0001-5787-9130"
+- family-names: "Swenson"
+  given-names: "David W.H."
+- family-names: "Henry"
+  given-names: "Michael M."
+  orcid: "https://orcid.org/0000-0002-3870-9993"
+title: "The Open Free Energy Example Notebooks"
+version: 0.1
+date-released: 2022-03-28
+url: "https://openfree.energy/"
+license: MIT License
+repository-code: "https://github.com/OpenFreeEnergy/ExampleNotebooks"