Translate README to Japanese

[TheStack-ai]

Context

README exists in Korean and English. Japanese translation would expand the user base.

What to do

Create README.ja.md based on README.en.md, add link in other READMEs.

Difficulty: Easy

No code knowledge needed.

[hparihar-07]

Hey I would like to work on this one

[TheStack-ai]

Thanks @hparihar-07! Feel free to go ahead — please base the translation on README.en.md and add a link to the new README.ja.md in the other READMEs. Open a PR when ready.