Add a section to the docs about implementing a Module-based supervisor (#230)

Author: axelson

lib/consumer_supervisor.ex

@@ -44,6 +44,10 @@ defmodule ConsumerSupervisor do
 
       defmodule Printer do
         def start_link(event) do
+          # Note: this function must return the format of `{:ok, pid}` and like
+          # all children started by a Supervisor, the process must be linked
+          # back to the supervisor (if you use `Task.start_link/1` then both
+          # these requirements are met automatically)
           Task.start_link(fn ->
             IO.inspect({self(), event})
           end)