add reference

Author: drbenvincent

docs/source/notebooks/event_study_pymc.ipynb

@@ -222,3 +222,14 @@ @article{GriffithCollider
   year={2020},
   publisher={Nature}
 }
+
+@article{sun2021estimating,
+  title={Estimating dynamic treatment effects in event studies with heterogeneous treatment effects},
+  author={Sun, Liyang and Abraham, Sarah},
+  journal={Journal of econometrics},
+  volume={225},
+  number={2},
+  pages={175--199},
+  year={2021},
+  publisher={Elsevier}
+}