Great question! Simply create a file called init.R, and include instructions for package installation. For example, this will install dplyr and collidr:

libraries_to_install <- c("dplyr", "collidr")
install_if_missing = function(p) {
  if (p %in% rownames(installed.packages()) == FALSE) {
    install.packages(p)
  }
}
invisible(sapply(libraries_to_install, install_if_missing))