add reference to Description field

[tercer]

From Uwe at CRAN

For future submissions: Is there some reference about the method you can add in the Description field in the form Authors (year) doi:.....?

[tercer]

From https://cran.r-project.org/web/packages/policies.html

Citations in the ‘Description’ field of the DESCRIPTION file should be in author-year style, followed by a DOI or ISBN for published materials, or a URL otherwise. Preferably, the year and identifier would be enclosed, respectively, in parentheses and angle brackets

Looks like these citations are becoming desired.

[tercer]

in INST/CITATION we're still using Imai, King, Lau. 2008. Journal of Computational Graphics and Statistics.

https://github.com/IQSS/Zelig/blob/f16809bdd3fde77cd51d5bdf99f533b421e7cf52/inst/CITATION#L22-L35