add demo docs

demo/kitchen-sink/docs/erlang.erl

@@ -0,0 +1,20 @@
+  %% A process whose only job is to keep a counter.
+  %% First version
+  -module(counter).
+  -export([start/0, codeswitch/1]).
+ 
+  start() -> loop(0).
+ 
+  loop(Sum) ->
+    receive
+       {increment, Count} ->
+          loop(Sum+Count);
+       {counter, Pid} ->
+          Pid ! {counter, Sum},
+          loop(Sum);
+       code_switch ->
+          ?MODULE:codeswitch(Sum)
+          % Force the use of 'codeswitch/1' from the latest MODULE version
+    end.
+ 
+  codeswitch(Sum) -> loop(Sum).