Basically the same as #32, but now for XML comments.

In some cases it is usefull to inject comments, for example about the time and memory it took to generate a sitemap. This can currently be done using a custom processing instruction, but a comment is a more elegant approach...

$output->addProcessingInstruction('comment', sprintf('generationTime: %f seconds', $generationTime));

If agreed that this is a reasonable request, I can try to come up with a PR.