Add Sant'Anna & Zhao (2020) reference for doubly robust DiD

Author: claude

README.md

@@ -1340,6 +1340,8 @@ This library implements methods from the following scholarly works:
 
 - **Callaway, B., & Sant'Anna, P. H. C. (2021).** "Difference-in-Differences with Multiple Time Periods." *Journal of Econometrics*, 225(2), 200-230. [https://doi.org/10.1016/j.jeconom.2020.12.001](https://doi.org/10.1016/j.jeconom.2020.12.001)
 
+- **Sant'Anna, P. H. C., & Zhao, J. (2020).** "Doubly Robust Difference-in-Differences Estimators." *Journal of Econometrics*, 219(1), 101-122. [https://doi.org/10.1016/j.jeconom.2020.06.003](https://doi.org/10.1016/j.jeconom.2020.06.003)
+
 - **Sun, L., & Abraham, S. (2021).** "Estimating Dynamic Treatment Effects in Event Studies with Heterogeneous Treatment Effects." *Journal of Econometrics*, 225(2), 175-199. [https://doi.org/10.1016/j.jeconom.2020.09.006](https://doi.org/10.1016/j.jeconom.2020.09.006)
 
 - **de Chaisemartin, C., & D'Haultfœuille, X. (2020).** "Two-Way Fixed Effects Estimators with Heterogeneous Treatment Effects." *American Economic Review*, 110(9), 2964-2996. [https://doi.org/10.1257/aer.20181169](https://doi.org/10.1257/aer.20181169)