Update README.md

add DOI to citation

Author: josesho

README.md

@@ -82,7 +82,7 @@ Please refer to the official [tutorial](https://acclab.github.io/DABEST-python-d
 
 *Joses Ho, Tayfun Tumkaya, Sameer Aryal, Hyungwon Choi, Adam Claridge-Chang*
 
-Nature Methods 2019, 1548-7105
+Nature Methods 2019, 1548-7105. [10.1038/s41592-019-0470-3](http://dx.doi.org/10.1038/s41592-019-0470-3)
 
 [Paywalled publisher site](https://www.nature.com/articles/s41592-019-0470-3); [Free-to-view PDF](https://rdcu.be/bHhJ4)