Document restore function (#10)

readme.md

@@ -33,6 +33,10 @@ The `skybox` handle can be used to perform various actions:
 `skybox.set(player, number)`
  -- Sets the skybox to the `number` in the list of current skyboxes.
 
+`skybox.restore(player)`
+ -- Reverts the player skybox to the last `skybox.set()` value.
+ -- Other skybox mods can properly restore the player's custom skybox.
+
 `skybox.add(skyboxdef)`
  -- Add a new skybox with skyboxdef to the list of available skyboxes.