The idea is simple: show a little text instead of empty content when the editor loads for the first time, and make it disappear automatically when the content gets focus.
There are two main problems to do this correctly: find out when the editor is focused or blurred and avoid the placeholder text when the content is read by other scripts.
In theory to get the focus status it should be enough to handle the 'focus' and 'blur' events of the editor instance, but one problem is that when a dialog opens the content fires a blur, but in in this situation we don't want to insert again the placeholder text, so the event listener checks if a dialog has been opened before inserting the text.
Passing that check we must verify if the content is "empty" so we get the raw data of the editor and test it again "empty" strings like "<br>" "<p> </p>" and the like that can be generated by different browsers.
As I didn't want to handle yet another plugin I've included it in the configuration Helper plugin and released as version 1.1. Also, the correct name for this plugin is already being used by a plugin that ships with the default CKEditor install but that it's totally unrelated to this feature that plugin resembles more the mail merge fields from MS Word, but at the time that plugin was created I don't think that HTML5 existed.