Add documentation for ActionMailbox

[benoittgt]

Hello

While working on rspec/rspec.github.io#143 I noticed that we do not have any documentation for action_mailbox.

PR for action_mailbox on rspec_rails #2119

I looked at a way to add it to our generated app. Is the best way to do it, to use a separate file like for https://github.com/rspec/rspec-rails/blob/master/example_app_generator/generate_action_mailer_specs.rb? (cc @JonRowe)

Any tips to do it properly welcome.

action_mailbox requires active_storage. I wrote a minimal bug report template to use action_mailbox and help me to tackle this issue on rspec-rails

Thanks

[JonRowe]

Seems like what you have in your bug report is similar to what we'd want in our spec generator? We typically scaffold a thing and make sure the scaffold tests work, so we'd want something similar?