Documentation for Migration Rollbacks

First thanks for the great project and the great documentation! :clap: :dancer:

The [docs for migrations](https://hexdocs.pm/distillery/guides/running_migrations.html) describe a pretty good approach. However, to the best of my search ability the docs seem not to mention how to do rollbacks of migrations which to me is an integral part of any deployment that I wouldn't want to launch without.

I hence propose adding it to the docs. I'm not an expert myself but the approaches from this blog posts seem very much inline with what is done for migrations in the distillery docs:

https://dev.to/vinhnglx/migration-scripts-migraterollbackseeding-database-when-releasing-a-phoenixotp-app-by-distillery-1f1h

Might also help people who go here looking for that information like me :)