In a mixer with the paddle attachment or in a mixing bowl, combine flour and sugar. Cut in shortening until texture of course meal. Add baking powder, baking soda, cinnamon, nutmeg, and salt. Slowly add buttermilk. Add eggs one at a time. Fold in rhubarb. Spoon into 2 greased muffin tins and bake in a 375-degree oven for 15 to 20 minutes.