Detailed write instructions are available here (long story short: you can write Raspberry Digital Signage as any other operating system for the Pi).
For image and video slideshow-based signaging, please have a look at Raspberry Slideshow.
Also, foyer is a (contributed and not supported by me) WordPress plugin that can be used for a ready-to-go media slider for Raspberry Digital Signage; an example here.
This “release” build of Raspberry Digital Signage is limited is some functionality: if you like this project please donate.
Docs / I have issues/questions