Update README.md

[kojix2]

This is an attempt to make README.md more concise and clear.

Work is in progress with the help of AI (mainly OpenAI's gpt-4.1).
However, it follows conservative procedures rather than the so-called Vibe Coding method.

1. a human (kojix2) checks README.md
2. human (kojix2) proposes changes to AI and confirms AI's opinion
3. the AI makes local edits
4. human (kojix2) checks the changes and commits only the good parts

[kojix2]

The editing process was not completed. In order to stay motivated, it seems that it is better to make small improvements repeatedly rather than making major changes.