More scaffolding for opening geojson file

[drelliche]

While some people posted that they had successfully opened mystery_file.geojson, it didn't seem like anyone had actually seen the map created by this file at the end of week 7.

Suggestion: add more scaffolding to this, with perhaps a link to https://geojson.io/ and a more explicit description of how you will know you have successfully opened it (i.e. you see a map).