add note in introduction

vignettes/introduction.Rmd

@@ -36,6 +36,8 @@ The statistical approach of **OutSeekR** centers around the use of five statisti
 
 Specifically, it uses the five statistics calculated on the observed data and compares the distributions of these statistics with counterparts calculated using simulated null data. Observed data yielding statistics more extreme than those of the null data suggest the presence of outliers.
 
+Note: Input data with low variation (e.g., where 50% or more of the values are identical) may cause issues in calculating the MAD or performing K-means clustering. It is recommended to exclude such data from the analysis to ensure reliable results.
+
 ## Overview
 
 As an overview, the **OutSeekR** algorithm does the following: