Update README.md

README.md

@@ -259,4 +259,12 @@ Data:
   doi = {10.1093/nar/gkac1008}
 }
 
+Optimizer:
+@article{wright2021ranger21,
+      title={Ranger21: a synergistic deep learning optimizer}, 
+      author={Wright, Less and Demeure, Nestor},
+      year={2021},
+      journal={arXiv preprint arXiv:2106.13731},
+}
+
 ```