I copied the unity run time files from here: spine-runtimes/spine-unity at master into my projects assets directory (so it looks like Assets/spine-unity).
Immediately I get an issue about the type TextureLoader and some other classes being missing.
There's some other missing things like Atlas, AtlasPage, etc, etc.
From googling I can't find anything about TextureLoader anywhere so I'm a bit lost now...
Ah looks like I need to import spine-runtimes/spine-csharp at master too or use the unity package on the site here.