Add package_json Gem requirement into migration doc

[GabrielLidenor]

Summary

package_json is a required gem for react_on_rails, if you don't have it in your Gemfile you will face the following error:

/Users/your_user/.rbenv/versions/3.3.6/lib/ruby/3.3.0/bundled_gems.rb:69:in require': cannot load such file -- package_json (LoadError)

Other Information

N/A

Pull Request checklist

• [ ] Add/update test to cover these changes
• [X] Update documentation
• [ ] Update CHANGELOG file