A Dance With Dragons left several story lines open that impact many of the characters and left a major cliffhanger at the end of the book. Winds of Winter is supposed to pick up right there and Martin should resolve all of these story lines, according to one source, he says that will happen early on.
Another source concludes that Winds of Winter will start about five minutes after the end of A Dance With Dragons on the eve of Iron Islanders surprise attack in Slaver’s Bay. That should be plenty of information for fans that are waiting to read the book with hopes of seeing the sixth season of Game of Thrones at a later time.
In the meantime, there will be plenty of time to read spoilers, see interviews and learn as much as you can before the book is published. If you watch Game of Thrones it might be better to finish season six, then pick up Winds of Winter.