title: Add a reference column migration in Rails 5
body: <p>A user has many uploads. I want to add a column to the uploads table that references the user. What should the migration look like?</p>
  <p>Related question for Rails 3: <a href="https://stackoverflow.com/questions/4954969/rails-3-migrations-adding-reference-column">Rails 3 migrations: Adding reference column?</a></p>
  <p>Related question for Rails 4: <a href="https://stackoverflow.com/q/22815009/972128">Add a reference column migration in Rails 4</a></p>
  <p>Related question for Rails 6: <a href="https://stackoverflow.com/q/60080860/how-to-add-reference-column-migration-in-rails-6-with-sqlite">How to add reference column migration in Rails 6 with SQLite</a></p>