Lore summary
Horde-aligned Skyborne 'embrace elemental traditions' and gain Shaman (Official). Windshapers are shamanistic spiritual leaders (wiki). See High Order entry for shared lore.
Start-zone changes
New race. Windshapers = shamanistic spiritual leaders aligned with the Horde; Shaman is Horde-only for Skyborne (both factions: Warrior, Hunter, Rogue, Druid).
Confidence note
Official (Blizzard) (race/classes/purchase) + Confirmed (multiple credible press, BlizzCon panel/demo) (racials)