Add note about using a provider that needs an API key

Author: Nabeel Shahzad

lib/js/base_map.js

@@ -46,7 +46,10 @@ const createMap = (opts) => {
     // Want to customizer the basemap? Look at the leaflet-providers page:
     //  http://leaflet-extras.github.io/leaflet-providers/preview/
     // Just set the provider name that you want below, or add additional
+    //
     // Some providers require an API key, so just look out for that
+    // If you want to use one of those, comment out the line below, and
+    // put the "Plain JavaScript" code they specify, and then use your API key
     L.tileLayer.provider(opts.provider).addTo(map);
 
     return map;