Add a README in Japanese

[Effeilo]

Description

browserux.css currently has a README in English and French.
Adding a Japanese translation would make the project more accessible to the Japanese developer community.

Task

Translate README.md into Japanese and save it as ja/README.md, following the same structure as the existing fr/README.md.

Acceptance criteria

• File created at ja/README.md
• All sections translated (Features, Installation, Documentation, Credits)
• Links adjusted to point to the correct relative paths
• Language switcher updated at the top: EN | FR | JA

Notes

No technical CSS knowledge required. This is a great first contribution for Japanese-speaking developers.