docs: add Japanese README

[eltociear]

Description

I created Japanese translated README.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• npm run test (following the project's testing guidelines)
• Manual testing with the following configuration:

Test Configuration:

• Client: (e.g., Cursor, Claude Code)
• OS: (e.g., macOS, Windows, Linux)
• Node.js version:

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md, CHANGELOG.md)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have checked to ensure my PR is focused on a single feature/fix
• I have updated the version number in package.json