Updated docs for using non-mapbox sourced tiles (#730)

docs/get-started/mapbox-tokens.md

@@ -12,9 +12,29 @@ But we would recommend using something like [dotenv](https://github.com/motdotla
 
 ## Display Maps Without A Mapbox Token
 
-It is possible to use the map component without the paid service, if you host your own map tiles. You will need a custom Mapbox style that points to your own [vector tile source](https://www.mapbox.com/mapbox-gl-js/style-spec/#sources-vector), and pass it to `ReactMapGL` using the `mapStyle` prop.
+It is possible to use the map component without the Mapbox service, if you use another tile source (for example, if you host your own map tiles). You will need a custom Mapbox GL style that points to your own [vector tile source](https://www.mapbox.com/mapbox-gl-js/style-spec/), and pass it to `ReactMapGL` using the `mapStyle` prop. This custom style must match the schema of your tile source. 
+
+Open source tile schemas include: 
+
+- [TileZen schema](https://tilezen.readthedocs.io/en/latest/layers/)
+- [OpenMapTiles schema ](https://openmaptiles.org/schema/)
 
 Some useful resources for creating your own map service:
 
 - [Mapbox Vector Tile Spec](https://www.mapbox.com/developers/vector-tiles/)
 - [Open source tools](https://github.com/mapbox/awesome-vector-tiles)
+
+If you are using a third party service that requires header based authentication, you can do this by defining a function to pass to `ReactMapGL` using the `transformRequest` prop.
+
+An example function:
+
+```js
+const transformRequest = (url, resourceType) => {
+        if (resourceType === 'Tile' && url.match('yourTileSource.com')) {
+            return {
+                url: url,
+                headers: { 'Authorization': 'Bearer ' + yourAuthToken }
+            }
+        }
+    }
+```