Improve the "Gem development" section of README?

[shouichi]

It is instructed to run bin/setup but it does not always work (depends on the ruby version). For example, with ruby 3.1.x, it does not work because the matrix gem was removed. Instead bundle exec appraisal should be used.

https://github.com/bensheldon/good_job#gem-development

Also, it might be a good idea to move this entire section to "CONTRIBUTING file".

https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

Thank you.

[bensheldon]

@shouichi thank you for the feedback and suggestions! I'll work on improving this section.

[shouichi]

Thank you! Feel free to close this when you think it's done 👍