The official Pokémon movie website has updated. Translations follow:

A Comic Adaptation of the Movie's Story "Pocket Monsters the Movie: I Choose You!" is Starting Serialization at Shogakukan's Manga App "Manga One"!

The story of "Pocket Monsters the Movie: I Choose You!", which premieres Saturday July 15th 2017, is being turned into a manga!
The serialization on Shogakukan's comic app "Manga One" will start prior to the movie's premiere,
on midnight Sunday July 2nd!
The serialization of a massive depiction of everything from Satoshi and Pikachu's meeting,
up to the last touching scene, will finally begin.
Go here for further information: http://manga-one.com/

©Nintendo･Creatures･GAME FREAK･TV Tokyo･ShoPro･JR Kikaku
©Pokémon ©2017 Pikachu Project　©Ryo Takamisaki/Shogakukan

Source: http://www.pokemon-movie.jp/news/?p=2850