Add aggregate.sf example

Demonstrates what happens when features in y do not intersect with x

R/aggregate.R

@@ -26,6 +26,10 @@
 #' p_buff = st_buffer(p, 0.2)
 #' plot(p_buff, add = TRUE)
 #' aggregate(p_buff, pol, mean) # increased mean of second
+#' # with non-matching features
+#' m3 = cbind(c(0, 0, 0.1, 0), c(0, 0.1, 0.1, 0))
+#' pol = st_sfc(st_polygon(list(m1)), st_polygon(list(m2)), st_polygon(list(m3)))
+#' aggregate(p, pol, mean)
 #' @export
 aggregate.sf = function(x, by, FUN, ..., do_union = TRUE, simplify = TRUE,
 		join = st_intersects) {