add decorate reader warning (#1596)

The bytes read from the reader are returned to a pool and
must not be changed by the decorated reader.

server.go

@@ -226,6 +226,7 @@ type Server struct {
  // If NotifyStartedFunc is set it is called once the server has started listening.
  NotifyStartedFunc func()
  // DecorateReader is optional, allows customization of the process that reads raw DNS messages.
+ // The decorated reader must not mutate the data read from the conn.
  DecorateReader DecorateReader
  // DecorateWriter is optional, allows customization of the process that writes raw DNS messages.
  DecorateWriter DecorateWriter