document application_record work around

[catmando]

1. Add this file to app/models/application_record.rb

# app/models/application_record.rb
# the presence of this file prevents rails migrations from recreating application_record.rb see https://github.com/rails/rails/issues/29407
require 'models/000_application_record.rb'

2. Then your real application_record should be named 000_application_record.rb, and placed in hyperloop/models directory.

 # app/hyperloop/models/000_application_record.rb
class ApplicationRecord < ActiveRecord::Base
   # whatever you want
end

This accomplishes 2 things
First - the app/models/application_record.rb prevents rails generators from recreating it. If you just move the application record to app/hyperloop/models, then every time you use the rails generator to create a model, it will think application record does not exist, and add another one!