Add CITATION.cff file

CFF is a file format for citations.¹ This adds a "Cite this repository"
button on the GitHub repo homepage.²

Values retrieved from the JOSS paper page.

¹ https://citation-file-format.github.io
² https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files

CITATION.cff

@@ -0,0 +1,45 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+
+preferred-citation:
+  type: article
+  title: "Augur: a bioinformatics toolkit for phylogenetic analyses of human pathogens"
+  doi: "10.21105/joss.02906"
+  journal: "Journal of Open Source Software"
+  year: 2021
+  month: 1
+  volume: 6
+  issue: 57
+  start: 2906
+  end: 2906
+
+  authors:
+    - family-names: Huddleston
+      given-names:  John
+
+    - family-names: Hadfield
+      given-names:  James
+
+    - family-names: Sibley
+      given-names:  Thomas R.
+
+    - family-names: Lee
+      given-names:  Jover
+
+    - family-names: Fay
+      given-names:  Kairsten
+
+    - family-names: Ilcisin
+      given-names:  Misja
+
+    - family-names: Harkins
+      given-names:  Elias
+
+    - family-names: Bedford
+      given-names:  Trevor
+
+    - family-names: Neher
+      given-names:  Richard A.
+
+    - family-names: Hodcroft
+      given-names:  Emma B.

README.md

@@ -40,6 +40,8 @@ Try out an analysis of real virus data by [completing the Zika tutorial](https:/
 
 Huddleston J, Hadfield J, Sibley TR, Lee J, Fay K, Ilcisin M, Harkins E, Bedford T, Neher RA, Hodcroft EB, (2021). Augur: a bioinformatics toolkit for phylogenetic analyses of human pathogens. Journal of Open Source Software, 6(57), 2906, https://doi.org/10.21105/joss.02906
 
+For other formats, refer to [CITATION.cff](./CITATION.cff).
+
 ## License and copyright
 
 Copyright 2014-2022 Trevor Bedford and Richard Neher.