Add warning about different sdl2 packages
I was following the beginners tutorial but I was having trouble compiling the initial nicoboot example app that was created for me but I couldn't figure out why. It said it couldn't load `sdl2/sdl` even though I'd installed nico, its dependencies and added SDL2.dll to the root folder.

I later figured out I had other nimble sdl2 packages installed (I think it was just called sdl2), while nico needs `sdl2_nim` specifically it appears. After uninstalling the old sdl2 packages using nimble it worked fine again.

Is there a warning somewhere about this or did I just miss it? Maybe it should be added to the README if not already there.