Also maybe a namespace. I doubt they're the only person to have ever made a class called `\Client` :)
Though I notice that the README states "[i]ts goal [is] to have one easy to use PHP file without dependencies", and they point out there's already a proper package out there, kelunik/acme. If the goal is a single file with no dependencies, I can understand the lack of Composer, as it's not really targeted at that audience.