Corelist

New Using core-list guide.

Thanks.
mem
I don't think it's correct to say the things that get rendered are template elements. (And we haven't referred to the template used to specify the item format yet). I feel like I'd like to have a high-level view of what the element does before the caveats.

Maybe should say something like, "When using a `<core-list>`, you provide a list of JavaScript objects (the data model), and a `<template>` element that defines how to render a single item. The `<core-list>` creates enough copies of the template to fill the viewport. As the user scrolls, these "virtual" items are recycled to display the items in the data model."