title: Add some basic markers to a map in mapbox via mapbox gl js
body: <p>I have a map styled with mapbox studio, however I'm having difficulty adding even a basic marker to it, however text is appearing where the marker should be which suggests that the marker would be there.</p>
  
  <p>So here's the code with that map style:</p>
  
  <pre><code>mapboxgl.accessToken = 'pk.eyJ1Ijoic21pY2tpZSIsImEiOiJjaWtiM2JkdW0wMDJudnRseTY0NWdrbjFnIn0.WxGYL18BJjWUiNIu-r3MSA';
  var map = new mapboxgl.Map({
      container: 'map',
      style: "mapbox://styles/smickie/cikb3fhvi0063cekqns0pk1f1",
      center: [-30.50, 40],
      zoom: 2,
      interactive: false
  });
  </code></pre>
  
  <p>And here some markers being added from an example in the api:</p>
  
  <pre><code>map.on('style.load', function () {
  
      map.addSource("markers", {
          "type": "geojson",
          "data": {
              "type": "FeatureCollection",
              "features": [{
                  "type": "Feature",
                  "geometry": {
                      "type": "Point",
                      "coordinates": [-77.03238901390978, 38.913188059745586]
                  },
                  "properties": {
                      "title": "Mapbox DC",
                      "marker-symbol": "monument"
                  }
              }, {
                  "type": "Feature",
                  "geometry": {
                      "type": "Point",
                      "coordinates": [-122.414, 37.776]
                  },
                  "properties": {
                      "title": "Mapbox SF",
                      "marker-color": "#ff00ff"
                  }
              }]
          }
      });
  
      map.addLayer({
          "id": "markers",
          "type": "symbol",
          "source": "markers",
          "layout": {
              "icon-image": "{marker-symbol}-15",
              "text-field": "{title}",
              "text-font": ["Open Sans Semibold", "Arial Unicode MS Bold"],
              "text-offset": [0, 0.6],
              "text-anchor": "top"
          }
      });
  });
  </code></pre>
  
  <p>However only the text and not the icons appear.</p>
  
  <p>Question is: how would I add just a normal basic colored marker to this map, not even one of the special icon ones?</p>
  
  <p>Thanks.</p>