Mention vignettes

visualize.Rmd

@@ -203,6 +203,10 @@ If you get an odd result, double check that you are calling the aesthetic as its
     ggplot(data = mpg) + 
       geom_point(mapping = aes(x = displ, y = hwy, color = "blue"))
     ```
+    
+1.  Vignettes are long-form guides the documentation things about 
+    a package that affect many functions. ggplot2 has two vignettes
+    what do they describe?
 
 ## Geoms