Minecraft wiki entry for Development_versions, associated text: Added an angle parameter for setting the default facing angle of a respawning player.
Syntax:
/spawnpoint [<targets>] [<pos>] [<angle>]
/setworldspawn [<pos>] [<angle>]
angle: Floating point yaw angle in degrees. Supports the relative ~ modifier, which defaults to the direction the executing player is facing.