add comment macro

It would be helpfull to have a xwiki comment mechanism.
A comment Macro rendered in a nice way in the WYSIWYG Editor is a good way to give the editor of a document some hints/guidelines specific for this page.

Currently a workaround is:

{{velocity}}
## my comment
{{/velocity}}

Problem is that the content of this comment isn't visible in the WYSIWYG Editor.