Update README.md

Reference paper listed

README.md

@@ -48,6 +48,11 @@ Documentation
 * [View Demo](https://github.com/PCMDI/pcmdi_metrics/blob/main/doc/jupyter/Demo/README.md)
 
 
+**Reference**
+
+Lee, J., P. J. Gleckler, M.-S. Ahn, A. Ordonez, P. Ullrich, K. R. Sperber, K. E. Taylor, Y. Y. Planton, E. Guilyardi, P. Durack, C. Bonfils, M. D. Zelinka, L.-W. Chao, B. Dong, C. Doutriaux, C. Zhang, T. Vo, J. Boutte, M. F. Wehner, A. G. Pendergrass, D. Kim, Z. Xue, A. T. Wittenberg, and J. Krasting, 2024: Systematic and Objective Evaluation of Earth System Models: PCMDI Metrics Package (PMP) version 3. Geoscientific Model Development (_accepted, publication in progress_) [[preprint](https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2720/)]
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+
 Contact
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