feat: add citation

Author: CristianoPizzamiglio

tensorflow_datasets/datasets/pneumoniamnist/CITATIONS.bib

@@ -1 +1,10 @@
-// TODO(pneumoniamnist): BibTeX citation
+@article{yang2023medmnist,
+  title={Medmnist v2-a large-scale lightweight benchmark for 2d and 3d biomedical image classification},
+  author={Yang, Jiancheng and Shi, Rui and Wei, Donglai and Liu, Zequan and Zhao, Lin and Ke, Bilian and Pfister, Hanspeter and Ni, Bingbing},
+  journal={Scientific Data},
+  volume={10},
+  number={1},
+  pages={41},
+  year={2023},
+  publisher={Nature Publishing Group UK London}
+}