Add user facing instructions for CSV upload of multiple items url field

[taylor-steve]

When uploading multiple items via CSV it appears that items with a blank url field are skipped, but if it is set to ~ you can effectively skip the field but still save and index the item. This is documented in the spec, but maybe not in any user facing way? It tripped @dnoneill and I up for a few minutes.

It might be worth adding a note or an example row in the downloadable CSV template?

spotlight/app/jobs/spotlight/add_uploads_from_csv.rb

Line 49 in c867eec

resource.build_upload(remote_image_url: url) unless url == '~'

spotlight/spec/jobs/spotlight/add_uploads_from_csv_spec.rb

Line 12 in c867eec

{ 'url' => '~' }