Improve the README

## Do you have a favorite Git Resource?
Add it to the README!
If you need help, check out the Appendix in the training manual.

Open the README and review its current resource content; consult the Appendix in the training manual if you need guidance. Add a favorite Git resource to the README, and consider the task done when the resource is included clearly without disrupting the existing document.

索引モデルが issue の本文から書いたものです。