Robinhood Markets Inc (NASDAQ:HOOD) and AppLovin Corp (NASDAQ:APP) announced they will join the S&P 500 Index (SPX) before the market opens on Monday, Aug. 22. The former will substitute Caesars Entertainment (CZR), and the latter will take MarketAxess Holdings' (MKTX) spot. Below, let's take a look at how HOOD and APP are responding to the news.HOOD is up 14.7% at $116.16 at last check, pacing for its best single-day percentage gain since April 9 and trading just shy of its Aug. 12, record high of$117.70. ...