Make a note about threefolds

[johndoknjas]

Not really our responsibility, but we could make a note in one of the 'set position / make moves' docstrings (or readme) about how Stockfish won't be able to detect threefolds over multiple calls. So we could suggest to the user that if they need this, they should make all the moves of a game at once, using the make_moves_from_starting_position function.

Alternatively, we could try to keep track of all moves played ourselves, but move history isn't really our responsibility.