Added pawn placement, fixed variable names and few errors in reference'

Changes
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1. Added size and pawns fields in class Board to store the pawns array and size passed to solve function

2. completed the snippet that adds pawns to the board

3. renamed the nbStates to boardArray for clarity (please feel free to revert to the previous if you don't find this intuitive)

4. Corrected few variable names in the checkRow, checkDiagonal and checkColumn methods. Corrected  the condition k < nbsquares to k < size * size in checkDiagonal