Added comment advising use of high-level API.

examples/scratchnetuser.py

@@ -5,6 +5,8 @@ Build a simple network from scratch, using mininet primitives.
 This is more complicated than using the higher-level classes,
 but it exposes the configuration details and allows customization.
 
+For most tasks, the higher-level API will be preferable.
+
 This version uses the user datapath and an explicit control network.
 """