I didn’t find any documentation on this, will happily write it.
For a start I had all my classes in one file and just included that file. Now its time to manage this properly, put each class in an own file, and custom template files as well.
My idea is to not touch the vendor directory at all, e.g. having all my self-written classes in /src, all self-written templates in /templates.
But of course I want to avoid having to include a lot of class files at the top of each script but get them trough the autoloader. I found this promising article on the composer autoloader, is that the ATK way to do it?