Add to Relax-NG Tutorial: info on making comments

[ebeshero]

We don't show people how to write comments in Relax-NG and need to add that to the tutorial. We may want to add a few other pointers.
See stuff posted

• http://dh.obdurodon.org/relaxng_notes.xhtml
• http://dh.obdurodon.org/relaxng2.html