Provide a `fetch` example

[xiaohutai]

Here's an example:

Remove the dummy response from index.html, and use the fetch code below instead of the response.forEach.
Update: make a GeoJSON example instead.

location.json

[
    {
        lat: 52.0866,
        lng: 4.2956,
        description: "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris sed vehicula mauris.",
        link: "https://www.vredespaleis.nl",
        title: "Peace Palace"
    },
    {
        lat: 52.0808905,
        lng: 4.3063686,
        description: "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris sed vehicula mauris.",
        link: "https://www.koninklijkhuis.nl",
        title: "Noordeinde Palace"
    }
];

fetch('locations.json')
    .then(function(response) {
        return response.json();
    })
    .then(function(json) {
        json.forEach(function(item){
            let location = new L.LatLng(item.latitude, item.longitude)
            // Spread syntax
            // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax
            let options  = {
                icon: icon,
                ...item
            };

            // Leaflet Marker
            // https://leafletjs.com/reference-1.3.2.html#marker
            let marker = new L.marker(location, options);

            // Template literals
            // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals
            marker.bindPopup(`
                <h2>${item.title}</h2>
                <p>${item.description}</p>
                <a href='${item.link}'>Read more</a>
            `);
            markers.addLayer(marker);
        }
    });
});