This commit adds an introductory section telling the reader how the
migration process works.

It also adds a last section telling the reader to remove any ineligible
adjustments from their database.

Lastly, it corrects the location of Spree configuration from the
`solidus_promotions` initializer to the `spree.rb` initializer.

We don't want to include `solidus_promotions` as a dependency in
`solidus` yet, but we do want to release the gem, so it should appear in
the all the lists of Solidus sub-gems.

This code has been automatically generated by our 'Prepare release' GitHub
action.

The actual release is not part of the automation, and it still needs to be manually done by a maintainer.

Due to the top level constant in testing.rake the release
task was also trying to release frontend. Using a local
variable instead.

This code has been automatically generated by our 'Prepare post-release' GitHub
action.

Make sure that:

- [ ] The new release has been published, along with its tag. See https://github.com/solidusio/solidus/releases/tag/v4.4.0
- [ ] The corresponding patch branch exists. See https://github.com/solidusio/solidus/tree/v4.4
- [ ] The corresponding backport-v4.4 label exists. See https://github.com/solidusio/solidus/labels