Add example to combine for missing = TRUE

[1beb]

Example needed for combine where the goal is to set the category(ies) to missing:

ds$BIDBAT2C <- crunch::combine(
  ds$BIDBAT2,
  name = "Grid (Collapsed)",
  combinations = list(
    list(
      name = "True", 
      categories = c("Mostly true", "More true than false")
    ),
    list(
      name = "False", 
      categories = c("Mostly false", "More false than true")
    ),
    list(name = "zzz", categories = "Not sure", missing = TRUE)
  )