doc: add Citation

Author: AlFontal

CITATION.bib

@@ -0,0 +1,10 @@
+@article{fontal2021climatic,
+  title={Climatic signatures in the different COVID-19 pandemic waves across both hemispheres},
+  author={Fontal, Alejandro and Bouma, Menno J and San-Jos{\'e}, Adri{\`a} and L{\'o}pez, Leonardo and Pascual, Mercedes and Rod{\'o}, Xavier},
+  journal={Nature Computational Science},
+  volume={1},
+  number={10},
+  pages={655--665},
+  year={2021},
+  publisher={Nature Publishing Group}
+}