Obsolete composer.json in MW 1.22 RC0 and earlier
Probably not worth to be mentioned on the installation page but MW 1.22 RC0 and earlier versions were shipped (respecticely 1.21.x is still shipped) with a composer.json file in the wiki directory. In case this file still exists when trying to install SMW and other extensions with composer it can cause issues that make using composer impossible.
Yeah, right since it is a MW problem and not a SMW problem it should not be added to the instructions. Additionally it only affects alphas, betas and rcs of MW 1.22. However it is good to have it on the talk page here for others to see. The only thing I wonder about is that you need to create a "composer.json" in that very spot for MW 1.22 anyway? I will not be working with MW 1.22 before probably May or June so I currently cannot check for myself.