Real Madrid or Mbappe haven't announced the news officially though as far as I can see. But after the celebrations they would also surely announce. I trust Fabrizio Romano as a source as his news are correct nearly every time. 
Anyways, from now on I'm looking forward to watching Mbappe in Real Madrid jersey. He is really hungry for Champions League titles and much more meaningful domestic titles compared to the Ligue 1. 
Mbappe arrival at Real Madrid is just waiting for an official announcement from the club, he and Real Madrid have reached an agreement and every document has been signed according to the agreement of both parties. Real Madrid is in euphoria after winning the Champions League, the announcement of Mbappe's arrival will probably be made next Sunday. Anyways, from now on I'm looking forward to watching Mbappe in Real Madrid jersey. He is really hungry for Champions League titles and much more meaningful domestic titles compared to the Ligue 1. Mbappe has made big steps in his career, joining Real Madrid has been his dream for a long time and Real Madrid can realize their plans to bring him in. Now he can be considered a new Real Madrid player and the first player to be brought in to complete the squad next season on a free transfer.