update README and kata description
tdd: Minesweeper Attempt 01: 1 fails
tdd: Minesweeper Attempt 01: 1 passes
tdd: Minesweeper Attempt 01: 1 refactor (remove unused method, add null case and split test in two)
tdd: Minesweeper Attempt 01: 2 fails
tdd: Minesweeper Attempt 01: 2 passes
tdd: Minesweeper Attempt 01: 2 refactor (and add one more test)
tdd: Minesweeper Attempt 01: 3 fails
tdd: Minesweeper Attempt 01: 3 passes / refactor