In the basic rules both an attack or going into hiding counts as an action, of which you can only do one per turn. But in the demonstration video of the Starter Box in the very first fight the group gets ambushed by goblins who are hidden, shoot arrows, and then go into hiding again in the same turn. Does WotC play their own game wrong, or is there a rule I missed that allows somebody to hide after an attack?
On Page 58 of the Lost Mine of Phandelver in the D&D Starter Set. Goblins have a ability called Nifty Escape that allows them to disengage or hide as a bonus action.
Like D&D 4e the 5e monster stat block spells out any special abilities and exception to the normal rules.
The GM actually spells that out. He says "Goblins, because they are particularly stealthy, can use their bonus action to hide." Presumably, goblins have a racial feature that works like the Rogue's Cunning Action and allows them to use a bonus action to make a Hide action.