Add an example of onresponse hooks

Also fix the guide entry on hooks.

examples/error_hook/src/error_hook_app.erl

@@ -0,0 +1,24 @@
+%% Feel free to use, reuse and abuse the code in this file.
+
+%% @private
+-module(error_hook_app).
+-behaviour(application).
+
+%% API.
+-export([start/2]).
+-export([stop/1]).
+
+%% API.
+
+start(_Type, _Args) ->
+	Dispatch = cowboy_router:compile([
+		{'_', []}
+	]),
+	{ok, _} = cowboy:start_http(http, 100, [{port, 8080}], [
+		{env, [{dispatch, Dispatch}]},
+		{onresponse, fun error_hook_responder:respond/4}
+	]),
+	error_hook_sup:start_link().
+
+stop(_State) ->
+	ok.