add citation to README

Author: Aariq

README.md

@@ -1,9 +1,14 @@
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 # Leveling up your data visualization skills with ggplot2
 
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- [![License: CC BY 4.0](https://img.shields.io/badge/License-CC_BY_4.0-lightgrey.svg)](https://creativecommons.org/licenses/by/4.0/)
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+[![License: CC BY 4.0](https://img.shields.io/badge/License-CC_BY_4.0-lightgrey.svg)](https://creativecommons.org/licenses/by/4.0/)
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-Source code for materials for a data visualization workshop.
+Source code for materials for a data visualization workshop series.
+
+If you use or adapt these materials, please cite them as follows:
 
+> Scott E, Diaz R, Riemer K (2025).
+> “Leveling up your data visualization skills with ggplot2.” <https://cct-datascience.github.io/ggplot2-workshop-series/>.