Chip Somodevilla/GettyThere's really only one thing to wonder about now that Moody's has downgraded China's credit rating, and that's what it means for Janet Yellen and the Federal Reserve.First, a refresher in case this is how you're getting the news. For the first time since 1989, Moody's — a debt ratings agency — downgraded China's credit rating from A1 from Aa3.