Hi,
Assuming your site is using en-US for the default culture setting (which it does by default unless you have forced it to use a different default culture), then the template file is located at /Data/MessageTemplates/en-US-RegisterConfirmEmailMessage.config
You could edit that file and add additional text but then you will need to keep a backup copy and remember to restore it after each upgrade since the upgrade will overwrite that file.
At some point I plan to implement something more flexible for customizing the message templates but for now it is what it is.
Hope it helps,
Joe