Soak rice in 1/4-cup water for approximately 1 hour, then cook rice in same water until thick. Add skim milk. Cook slowly stirring occasionally until thickened, but stop cooking before all of the liquid is absorbed. Allow to cool, then add vanilla, sugar, and raisins. Fold in whipped topping. Spoon into 6 serving dishes. Garnish with sprig of mint, if desired.