Document Drush commands in README

Yeah those commands are not really meant to be used, they are just a way to intercept when a migration command is run to start the boost AFAIK.

Thanks for a fast response @nicxvan, in that case, maybe that could get a mention in the README?

PS. Great show with Andy Marquis the other day! For me, the killer feature of Custom Field is how easy it is to import multivalue fields with Migrate, compared to setting up a migration for Paragraphs: https://www.drupal.org/docs/extending-drupal/contributed-modules/contrib... →

Also, the ability to copy content from one field to another with https://www.drupal.org/project/field_updater_service → using Drush is fantastic, but somewhat buried in the "Quick start", where it looks like complicated update hooks are needed, when they're not.

Thanks! Merged!