What documentation exists for R?

Chapter 5: R Add-On Packages 74
Even more conveniently, you can install and automatically update packages from
within R if you have access to repositories such as CRAN. See the help page for
available.packages() for more information.
You can use several library trees of add-on packages. The easiest way to tell R to use
these is via the environment variable R_LIBS which should be a colon-separated list of
directories at which R library trees are rooted. You do not have to specify the default
tree in R_LIBS. E.g., to use a private tree in ‘$HOME/lib/R’ and a public site-wide tree in
‘/usr/local/lib/R-contrib’, put
R_LIBS="$HOME/lib/R:/usr/local/lib/R-contrib"; export R_LIBS
into your (Bourne) shell profile or even preferably, add the line
R_LIBS="~/lib/R:/usr/local/lib/R-contrib"
your ‘~/.Renviron’ file. (Note that no export statement is needed or allowed in this file;
see the on-line help for Startup for more information.)
5.3 How can add-on packages be used?
To find out which additional packages are available on your system, type
library()
at the R prompt.
This produces something like