Fix basic example

basic/README.md

@@ -1,5 +1,4 @@
-Basic example
----
+## Basic example
 
 Initialize a map with coordinates set by React, update the state of coordinates on map movement, and display that data in an element provided by the render function.
 
@@ -15,4 +14,10 @@ Initialize a map with coordinates set by React, update the state of coordinates
 
     open http://localhost:3000/
 
+### Access token
+
+    mapboxgl.accessToken = // YOUR_API_KEY_HERE
+
+Replace the [API key](https://docs.mapbox.com/help/getting-started/access-tokens/) in `src/Map.js`.
+
 Runs the app from http://localhost:3000