contrib.url adds the appropriate type-specific path within a
repository to each URL in
contrib.url(repos, type = getOption("pkgType"))
character vector, the base URL(s) of the repositories to use.
character string, indicating which type of packages: see
A character vector of the same length as
type = "both" this will use the source repository.
The ‘R Installation and Administration’ manual for how to set up a repository.