Create CITATION.cff to assist people in citing the repo

Author: tdunning

CITATION.cff

@@ -0,0 +1,10 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+- family-names: "Dunning"
+  given-names: "Ted"
+  orcid: "https://orcid.org/0000-0000-0000-0000"
+title: "t-digest"
+version: 3.2
+date-released: 2017-08-06
+url: "https://github.com/tdunning/t-digest"