doc: add references for spatstat

Author: ctoney

R/spatstat_helpers.R

@@ -66,7 +66,20 @@
 #' `factor` marks. If not already `factor`, these will be coerced as such upon
 #' input. `factor` marks are those that take only a finite number of possible
 #' values (e.g. colors or types).
+#' 
+#' @references
+#' Baddeley A, Rubak E, Turner R (2015). _Spatial Point Patterns: Methodology
+#' and Applications with R_. Chapman and Hall/CRC Press, London.
+#' ISBN 9781482210200,
+#' \url{https://www.routledge.com/Spatial-Point-Patterns-Methodology-and-Applications-with-R/Baddeley-Rubak-Turner/p/book/9781482210200/}.
 #'
+#' Ripley, B.D. (1977) Modelling spatial patterns. _Journal of the Royal
+#' Statistical Society: Series B (Methodological)_, **39**:2, 172–192.
+#' \url{https://doi.org/10.1111/j.2517-6161.1977.tb01615.x}.
+#' 
+#' @seealso
+#' \pkg{spatstat} package: \url{https://cran.r-project.org/package=spatstat}
+#' 
 #' @examples
 #' # observation window for the standard FIA plot design
 #' w <- create_fia_owin()