grains exercise: Split into introduction and instructions

exercises/grains/instructions.md

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+# Instructions
+
+Calculate the number of grains of wheat on a chessboard.
+
+A chessboard has 64 squares.
+Square 1 has one grain, square 2 has two grains, square 3 has four grains, and so on, doubling each time.
+
+Write code that calculates:
+
+- the number of grains on a given square
+- the total number of grains on the chessboard

exercises/grains/introduction.md

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+# Introduction
+
+There once was a wise servant who saved the life of a prince.
+The king promised to pay whatever the servant could dream up.
+Knowing that the king loved chess, the servant told the king he would like to have grains of wheat.
+One grain on the first square of a chessboard, with the number of grains doubling on each successive square.