Created sudokuGame.java, added basic snippets

Current issues:
Random number generation traps the grid filling into long loops.
Need to refine the way the number candidates are chosen to avoid
traps.

Added snippets:
public void sudokuGame(int difficulty)
-Constructor for the game grid
public void fillGrid()
-Fills the grid with numbers that respect the Sudoku
grid condition.
public boolean validateCell(int row, int col, int value)
-Checks if the value input can be inserted at (row,col) without breaking the Sudoku grid conditions