add lemon bar recipe (#30)

lemon-squares.md

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+Lemon squares
+======
+A tart tasty tempting treat
+
+## Ingredients
+* 1 cup butter, softened
+* ½ cup white sugar
+* 2 cups all-purpose flour
+* 4 eggs
+* 1 ½ cups white sugar
+* ¼ cup all-purpose flour
+* 2 lemons, juiced
+
+## Tools
+* whisk
+* 2 medium bowls
+* 9x13 inch pan
+
+## Steps
+1. Preheat oven to 350 degrees F (175 degrees C).
+1. In a medium bowl, blend together softened butter, 2 cups flour and 1/2 cup sugar. Press into the bottom of an ungreased 9x13 inch pan.
+1. Bake for 15 to 20 minutes in the preheated oven, or until firm and golden. In another bowl, whisk together the remaining 1 1/2 cups sugar and 1/4 cup flour. Whisk in the eggs and lemon juice. Pour over the baked crust.
+1. Bake for an additional 20 minutes in the preheated oven. The bars will firm up as they cool. For a festive tray, make another pan using limes instead of lemons and adding a drop of green food coloring to give a very pale green. After both pans have cooled, cut into uniform 2 inch squares and arrange in a checker board fashion.
+
+## Additional Notes
+* They should not brown on top
+
+## References
+* https://www.allrecipes.com/recipe/10294/the-best-lemon-bars/