docs: add Japanese versions of README.md and CONTRIBUTING.md

[eltociear]

I created Japanese translated documents.

[xile611]

@eltociear are you Japanese? Because I can't understand Japanese... I can't check the content of these document, I can only check the format of document.

[eltociear]

@xile611 Yes, I am.

[xile611]

@eltociear
We greatly appreciate your contribution to VChart during the previous quarter.
As part of our initiative to foster engagement in the VChart open source community, we offer quarterly rewards to recognize our contributors' efforts. Maybe you can send me an email, I'll invite you to join our contributors' Lark group where you can claim your reward.