Add DOIs to description field in DESCRIPTION.

DESCRIPTION

@@ -1,10 +1,13 @@
 Package: mully
 Type: Package
 Title: Create, Modify and Visualize Multi-Layered Networks
-Version: 2.1.22
+Version: 2.1.25
 Author: Zaynab Hammoud
 Maintainer: Zaynab Hammoud <[email protected]>
 Description: Allows the user to create graph with multiple layers. The user can also modify the layers, the nodes, and the edges. The graph can also be visualized.
+    Zaynab Hammoud and Frank Kramer (2018) <doi:10.3390/genes9110519>.
+    More about multilayered graphs and their usage can be found in our review paper:
+    Zaynab Hammoud and Frank Kramer (2020) <doi:10.1186/s41044-020-00046-0>.
 URL: https://github.com/frankkramer-lab/mully
 BugReports: https://github.com/frankkramer-lab/mully/issues
 License: GPL (>= 2)
@@ -20,4 +23,3 @@ Suggests:
     knitr,
     rmarkdown
 VignetteBuilder: knitr
-Reference: Zaynab Hammoud and Frank Kramer (2018) <https://doi.org/10.3390/genes9110519>