https://www.cnet.com/news/avengers-infinity-war-russo-brothers-beg-fans-not-to-spoil-marvel-plot/

Addressed "to the greatest fans in the world," the note says "only a handful of people know the film's true plot," and that years of work have gone into preserving the film's mystery. It requests that fans who see the film "don't spoil it for others, the same way you wouldn't want it spoiled for you." 

The tweet was hashtagged #ThanosDemandsYourSilence.