`responseRoot` specifies the value in the feed that holds the array of items
used to populate the store. 

For example, the Yelp feed returns a single object which contains a _businesses_
property whose value is the array of businesses. Therefore, the store that reads
it should specify 

`responseRoot: businesses`.

    {
    "businesses": [
        {
        "id": "nsw3tlB-hwz4KbPYcuaSww",
        "alias": "novanta-madison",
        "name": "Novanta",
        "image_url": "https://s3-media1.fl.yelpcdn.com/bphoto/2QtMzHr1ZNoflLzXKRza9A/o.jpg",
        "is_closed": false,
        "url": "https://www.yelp.com/biz/novanta-madison?adjust_creative=hAlq2S6c7S0nxZcdmjtkhA&utm_campaign=yelp_api_v3&utm_medium=api_v3_business_search&utm_source=hAlq2S6c7S0nxZcdmjtkhA",
        "review_count": 329,
        "categories": [
            {
            "alias": "pizza",
            "title": "Pizza"
            },
        ...