Update README.md

Author: Joseph Damiba

_posts/plotly_js/README.md

@@ -90,7 +90,8 @@ See additional options [HERE](https://github.com/plotly/documentation/blob/b5833
  - Try to use "real" data to make the examples realistic and useful for users. 
  - Avoid using random or dummy data as much as humanly possible! Should only be a last resort. 
  - Upload datafiles to https://github.com/plotly/datasets, as importing data rather than pasting a large chunk of data in the tutorial creates a cleaner example. 
- 
+ - Use `var config = {mapboxAccessToken: "your access token"};` if your chart requires Mapbox authentication. `"your access token` will replaced by Plotly's private token at build time. In development mode, you will need to create a `_data/mapboxtoken.yml` file and paste Plotly's non-URL restricted Mapbox key into it. This is available in 1Password.
+ 
 8. Add Thumbnail Images
  - Thumbnail images should named `your-tutorial-chart.jpg` and be *EXACTLY* 160px X 160px
  - posts in the following `display_as` categories **MUST** have a thumbnail