Update README.md

README.md

@@ -27,5 +27,7 @@
   * correctness
   * robustness
 
-### Reference 
+### References
+* Rezende, D. J., & Mohamed, S. (2015). [Variational Inference with Normalizing Flows.](https://arxiv.org/abs/1505.05770v6)
+* Kobyzev, I., Prince, S. J. D., & Brubaker, M. A. (2019). [Normalizing Flows: An Introduction and Review of Current Methods.](https://arxiv.org/abs/1908.09257v4)
 * This repository is inspired by [Eric Jang](https://github.com/ericjang/normalizing-flows-tutorial) and [TensorFlow probability docs.](https://www.tensorflow.org/probability)