docs(readme): add diffraxtra

Author: nstarman

README.md

@@ -63,7 +63,8 @@ If you found this library useful in academic research, please cite: [(arXiv link
 
 **Always useful**  
 [Equinox](https://github.com/patrick-kidger/equinox): neural networks and everything not already in core JAX!  
-[jaxtyping](https://github.com/patrick-kidger/jaxtyping): type annotations for shape/dtype of arrays.  
+[jaxtyping](https://github.com/patrick-kidger/jaxtyping): type annotations for shape/dtype of arrays.
+[diffraxtra](https://github.com/GalacticDynamics/diffraxtra): extras for `diffrax`; OOP and vectorization.
 
 **Deep learning**  
 [Optax](https://github.com/deepmind/optax): first-order gradient (SGD, Adam, ...) optimisers.