Adds a holosign, and a surgery one to get it started. Set up so it's easy enough to create another one, just make an icon, and add this to holosign.dm:

/obj/machinery/holosign/example
	name = "example holosign"
	desc = "Small wall-mounted holographic projector. This one reads EXAMPLE."
	on_icon = "example"

to holosign.dm.