[DOC] Create a good README.md

この issue には説明がありません。

README.md is the only named deliverable; inspect the repository structure, CMake configuration, and CLI-tool metadata before defining what the README must cover. Because the issue body gives no acceptance criteria, confirm the expected contents with maintainers; done means a clear README.md that accurately explains the project and its initial Git workflow.

索引モデルが issue の本文から書いたものです。