Add reference to the preprint.

README.md

@@ -2,11 +2,15 @@
 
 # Multi-Proxy, Multi-Season Streamflow Reconstruction with Mass Balance Adjustment
 
-Data and code to reproduce the entitled paper.
+Data and code to reproduce the entitled paper (Nguyen et al, 2020).
 
 Download the repo as a `zip` file and extract it to your working folder. If you use RStudio, open the file `multi-proxy-mbr.Rproj` to set up the project environment. Otherwise, you can set up your working folder manually.
 
 We encourage you to first open the file `multiproxy-mbr.html` to read through the analysis steps and corresponding results.
 
 To reproduce the results, you can then open the file `multiproxy-mbr.Rmd` and follow the steps. The file `R/init.R` lists the necessary packages, so you may want to install them before running the code.
 
+
+## Reference
+
+Nguyen, H. T. T., Galelli, S., Xu, C., & Buckley, B. (2020). Multi-Proxy, Multi-Season Streamflow Reconstruction with Mass Balance Adjustment. Earth and Space Science Open Archive, 22. https://doi.org/10.1002/essoar.10504791.1