clearer tokens docs for dev (#433)

Author: nicolaskruchten

README.md

@@ -41,6 +41,7 @@ This module's entry point is a React component called `<PlotlyEditor />` which c
 This repo contains a [dev app](https://github.com/plotly/react-chart-editor/tree/master/dev) that depends on the components locally and is configured for hot reloading, for easy local development. A `jest`-based test suite is also included.
 
 ```
+cp accessTokens.tpl.js accessTokens.js # and edit to taste
 npm install
 npm start
 # hacking happens here

accessTokens.tpl.js

@@ -0,0 +1,8 @@
+// To use Satellite Maps in the Editor, Mapbox access tokens are required.
+// see https://www.mapbox.com/help/how-access-tokens-work/
+
+const ACCESS_TOKENS = {
+  MAPBOX: '',
+};
+
+export default ACCESS_TOKENS;