Add YAML safe load documentation

[nazamoresco]

Hey,

We updated to Rails 6.1.6.1 in our project. For this upgrade, we had to define config.active_record.yaml_column_permitted_classes so the audited gem would work. We used the classes defined in the source code test's application.rb.

I think a warning in the documentation that this has to be defined for some Rails versions would be useful for developers using the gem. I can open a PR if this information belongs in the audited README.