Add details about exactly where to require the engine.

[phallstrom]

This clarifies exactly where to require goodjob in config/application.rb
and adds a note about how to resolve the issue when the dashboard won't
reload after saving a file.

Context: I skimmed the README when I set it up and made my
application.rb look like this:

require "active_model/railtie"
require "active_job/railtie"
require "active_record/railtie"
require "active_storage/engine"
require "action_controller/railtie"
require "action_mailer/railtie"
require "action_text/engine"
require "action_view/railtie"
require "action_cable/engine"
require "sprockets/railtie"

Bundler.require(*Rails.groups)

require "good_job/engine"

With this setup, the dashboard would work fine until I saved any file
(ie. a model). After that reloading the dashboard would give this error:

Routing Error
uninitialized constant GoodJob::ExecutionsController

By moving the require line before the Bundler.require line, this
problem went away.

[bensheldon]

Thank you for this Readme improvement! 🙌