EnsureVisible only works for SingleChildScrollView

[X-Wei]

Hi,

Great job providing this package !

I have had some problem with EnsureVisible, my initial implementation used a ListView, and the EnsureVisible widget will not auto-scroll and show.

After comparing my code to the example, I found out that I need to use a SingleChildScrollView+Column instead of a ListView to make it work: https://github.com/X-Wei/flutter_catalog/commit/ee8b9802bd29baa5e1cd0a887454eeab598cb944

If it's the intended behavior(seems so: https://stackoverflow.com/a/49154882/12421326), would be much nicer if we can highlight this in the documentation.