Corelist

New Using core-list guide.

Thanks.
mem
I wonder if this and the caveat about using images could come after an initial summary of how it's used. Maybe an executive summary of what to know when using core-list?

- Use `<core-list>` when working with lists large enough that ordinary data binding (`template repeat`) is impractical.
- The list automatically adjusts ...
- Can be issues with images ...