Explain how to create revisions with bulk actions

[dehnert]

The documentation notes:

Bulk actions, such as Queryset.update(), do not send signals, so won’t be noticed by django-reversion.

It would be helpful to expand that infobox with how people can work around that if desired and generate the versions anyway. In particular, I think looping over the queryset and using add_to_revision should work. Of course it would be good include a performance caveat, especially if #924 isn't fixed first.

Possible text:

Bulk actions (such as Queryset.update(), Queryset.bulk_update, or Queryset.bulk_create()) do not send signals, so won’t be noticed by django-reversion. You can use add_to_revision with bulk_update or bulk_create as a workaround, but this may eliminate the performance advantage of doing bulk actions since this requires fetching the objects into memory (unlike update()) and reversion will do one query per new version object (#924).

[etianen]

I'd take a PR that documented this. Your approach is sound, and the performance would still be better than performing mutiple save()s on the models.