A start at an introductory migration path for moving from Paperclip to
[ActiveStorage].

ActiveStorage is coming in Rails 5.2 (April, perhaps) and we want to
help people move from Paperclip to it. Having a file upload strategy
baked into Rails itself will allow us all to focus our efforts and
follow a common goal.

This migration guide is rudimentary at best: it works for a `User`
object with one `avatar` attachment. It is provided as a framework to
start the documentation.

I have tried this in a dummy app.

[ActiveStorage]: http://edgeguides.rubyonrails.org/active_storage_overview.html