GeoJSON needs .json file ext, but geojson format

Trying to mitigate end-user frustration by adding two small notes to clarify how to upload GeoJSON files. 

This instructs users to avoid uploading valid GeoJSON with a valid .geojson extension (CKAN requires .json), causing an internal server error on download and preview (see [CKAN-1534](https://github.com/ckan/ckan/issues/1534)), and instructs them to use .json extension in combination with "geojson" as the resource format.