add reference for PLS-DA method in trans_beta

R/trans_beta.R

@@ -87,7 +87,8 @@ trans_beta <- R6Class(classname = "trans_beta",
 		#' @param method default "PCoA"; "PCoA", "NMDS", "PCA", "DCA" or "PLS-DA".
 		#' 	  PCoA: principal coordinates analysis; NMDS: non-metric multidimensional scaling, PCA: principal component analysis; DCA: detrended correspondence analysis; 
 		#' 	  PLS-DA: partial least squares discriminant analysis.
-		#' 	  Please refer to the paper <doi:10.1111/j.1574-6941.2007.00375.x> for the details of the methods.
+		#' 	  For the methods details, please refer to the papers <doi:10.1111/j.1574-6941.2007.00375.x> (for PCoA, NMDS, PCA and DCA) and 
+		#' 	  <doi:10.1186/s12859-019-3310-7> (for PLS-DA).
 		#' @param ncomp default 3; dimensions shown in the results (except method "NMDS").
 		#' @param trans default FALSE; whether species abundance will be square transformed; only available when \code{method} is "PCA" or "DCA".
 		#' @param scale_species default FALSE; whether species loading in PCA or DCA is scaled.