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.

Start by opening the repository's README and review the Appendix in the training manual for guidance. Add one favorite Git resource to the README, preserving its existing organization and formatting; the issue is done when the resource appears there.

索引モデルが issue の本文から書いたものです。