One way to do it is to add a your text into the 'caption' metadata box in the Library to the first image in your show, but make sure that none of the other photos in the show have any captions, (unless you want them to show up). Then in the Slideshow editor, click on the 'ABC' in the bottom toolbar and from the dropdown options choose 'caption' from the list. In the editor every image will have either the text from the caption metadata showing, or will show as <empty>. You can drag this text around, change transparancy and font or resize as necessary using the text editor tools in the righthand slideshow panel. (You must have text overlay ticked.)
The slides with <empty> showing the in the editor will not have any text when you play the slideshow, but any with caption metadata will show! p178 of the LR PDF manaul has more info.
(I guess if you already have caption metadata in all your images, you may be able to create virtual copies, strip out all the unnecessary data and use those for creating the slideshow, but as a beginner to LR5 I haven't tried that as I'm still pretty lax about metadata as a rule!)
Hope that works for you,
Rob
