Update read the docs to explain format of digest file that is exported by GOPHER

[hansenp]

We could take this from the Diachromatic documentation:

https://diachromatic.readthedocs.io/en/latest/digest.html

[pnrobinson]

done @hansenp note that the documentation is not quite correct Please see the new page, Enrichment status was changed to Enrichment_status for consistency