Sudoku for beginners

This is a simple Sudoku with focus on the game:
- no ads,
- no timer,
- no sounds,
- no fancy distracting stuff,
- simply enjoy the game

It packs some features that are useful for beginners:
- several difficulty levels
- easiest level has a hint button (press it when you're stuck)
- colors for the numbers
- remaining numbers indicator
- note taking mode
- unlimited undo levels
- rules of the game are explain