add date field to description to fix call to citation

[nuest]

citation("sensorweb4R")

returns the warning

Warning message:
In citation("sensorweb4R") :
  kein Datumsfeld in der Datei DESCRIPTION des Paketes ‘sensorweby’

[autermann]

There is more missing than just a date (see Writing R Extensions). Apperently a inst/CITATION file is needed, that produces a BibTeX entry like this:

year <- sub("-.*", "", meta$Date)
note <- sprintf("R package version %s", meta$Version)

bibentry(bibtype = "Manual",
         title = "{nlme}: Linear and Nonlinear Mixed Effects Models",
         author = c(person("Jose", "Pinheiro"),
                    person("Douglas", "Bates"),
                    person("Saikat", "DebRoy"),
                    person("Deepayan", "Sarkar"),
                    person("R Core Team")),
         year = year,
         note = note,
         url = "http://CRAN.R-project.org/package=nlme")