Update README

Author: mbostock

README.md

@@ -748,7 +748,7 @@ Plot.barY(alphabet, {x: "letter", y: "frequency"}).plot({width: 1024})
 
 #### Plot.marks(...*marks*)
 
-A convenience method for composing a mark from a series of other marks. Returns an array of marks that implements the *mark*.plot function.
+A convenience method for composing a mark from a series of other marks. Returns an array of marks that implements the *mark*.plot function. See the [box mark implementation](./src/marks/box.js) for an example.
 
 ### Area