Fill in documentation

14. Update the documentation and add an example.

#' Split a string
#'
#' Takes the first result of the base::strsplit function.
#'
#' @param x A character vector with one element.
#' @param split What to split on.
#'
#' @return A character vector.
#' @export
#'
#' @examples
#' x <- "alfa,bravo,charlie,delta"
#' strsplit1(x, split = ",")