Add instructions for multistage exercise

slides/intro-fullday.yml

@@ -45,6 +45,10 @@ chapters:
       Let's update our Dockerfiles to leverage multi-stage builds!
 
       The code is at: https://bitbucket.org/jgarrouste/k8s-wordsmith-exo/src/master/
+
+      Use a different tag for these images, so that we can compare their sizes.
+
+      What's the size difference between single-stage and multi-stage builds?
 - - intro/Naming_And_Inspecting.md
   - intro/Labels.md
   - intro/Getting_Inside.md