Soon, Robinhood Markets Inc.’s retail traders won’t have to leave the platform to brag about their positions or post taunting memes celebrating their investment victories. The online brokerage is breaking into social media.The company will invite a small group of customers to join Robinhood Social early next year, then broaden the availability later, the company said in a statement Tuesday. Initially, all posts by traders will be required to include a trade of equities, options or other assets. Those positi ...