The Russo Brothers have also implicitly invited fans to watch Avengers: Infinity War very, very closely so they could potentially spot where this little Easter Egg is hidden.
Given the subject matter of Avengers: Infinity War one can surmise that this has something to do with either Thanos or the Infinity Gauntlet.
Beginning with their work on Captain America: Civil War, the Russo Brothers have been known for sharing cryptic tweets with minuscule teases at their upcoming film.
Avengers: Infinity War is the Russo Brothers’ third turn at directing a Marvel Studios movie, although their first time directing a movie under the Avengers banner (despite the fact that many people consider Civil War to be more of an Avengers movie than a Captain America movie).