Add software citation source

Not all contributors are authors.  Only those who have added
functional additions to the software (i.e., implementation of other
languages)

Author: jakewilliami

README.md

@@ -11,3 +11,7 @@
 
 ## Description
 This is a minimal package for a pure Julia implementation of converting numbers in their Arabic form to numbers in their Anglo-Saxan form.  The process of **spelling out** a number has also been referred to as **set full out**, to write out in **long hand**, and — as a great mind once suggested — to **transnumerate**.
+
+## Citation
+
+If your research depends on SpelledOut.jl, please consider giving us a formal citation: [`citation.bib`](./citation.bib).

citation.bib

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+@software{ireland2024spelledout,
+  title={{SpelledOut.jl}: A minimal, {Julia}-written tool for converting numbers to words},
+  author={Ireland, Jake W and Riedemann, Andr{\'e}s and  de Freitas Pereira, Gabriel},
+  year={2024},
+  howpublished={\url{https://github.com/jakewilliami/SpelledOut.jl}},
+  note={{Julia} package version 1.2.1}
+}