Add File Import example

[jameswilson]

Would be cool to add the file import example from this blog post to the repository:

https://www.mtech-llc.com/blog/ada-hernandez/how-migrate-images-drupal-8-using-csv-source

Ideally, it the import would be kept completely separate from the existing "people.csv" example. And since the example there is an import of also profile information, there is a bit of confusing overlap.

Maybe it would make sense to turn int into something like a CSV import of "article.csv" with an image field?