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.

Zelig/inst/CITATION

Lines 22 to 35 in f16809b

	bibentry(
	bibtype="Article",
	title = "Toward A Common Framework for Statistical Analysis and Development",
	author = c(
	person("Kosuke", "Imai"),
	person("Gary", "King"),
	person("Olivia", "Lau")
	),
	journal = "Journal of Computational Graphics and Statistics",
	volume = 17,
	number = 4,
	year = 2008,
	pages = "892-913",
	url = "https://gking.harvard.edu/files/abs/z-abs.shtml")