Simply add the Editor Comment block where you want to leave comments.
It takes any plain text you enter and only shows it again during edit mode. When the edit is saved and the page is shown normally, nothing is shown.
Edit the page again, and you can read the comment.
For those who want to customise the comment style, two constants can optionally be set to define colors.
Colors can be in any CSS acceptable format.
A further constant can be used to define a custom date/time format:
This constant accepts any php date/time format string and, when defined, will be used to format the date/timestamp shown with comments. The date/time format string defined in EDITOR_COMMENT_DATE_FORMAT is also passed through the t() translation method for sites using locale based date/time formats.
During development, you may want to actually show editoe comment blocks on site pages. This can be enabled globally by setting a configuration constant to true.