This retail favorite is joining a major index.Shares of Robinhood Markets (HOOD -2.29%) soared this week, finishing up 13.6%. The jump comes as the S&P 500 and the Nasdaq-100 gained 1.6% and 1.9%, respectively.The online-trading platform and favorite of retail traders saw its shares soar this week following the official announcement of its inclusion in the S&P 500.Robinhood joins the S&P 500, replacing CaesarsThe company will officially join the S&P 500 index before markets open on Sept. 22, taking the plac ...