docs: add Japanese README (#598)

I created Japanese translated README.
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> [!IMPORTANT]
> Add Japanese translation of README and update language links in
existing READMEs.
> 
>   - **Documentation**:
>     - Add `README-JP.md` for Japanese translation of the README.
> - Update language navigation links in `README.md` and `README-CN.md`
to include Japanese translation.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=julep-ai%2Fjulep&utm_source=github&utm_medium=referral)<sup>
for f6dc4d2. It will automatically
update as commits are pushed.</sup>

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README-CN.md

@@ -1,4 +1,4 @@
-<sup>[English](README.md) | 中文</sup>
+<sup>[English](README.md) | 中文 | [日本語](README-JP.md)</sup>
 
 <div align="center">
     <img src="https://socialify.git.ci/julep-ai/julep/image?description=1&descriptionEditable=Build%20AI%20agents%20and%20workflows%20with%20a%20simple%20API&font=Source%20Code%20Pro&logo=https%3A%2F%2Fraw.githubusercontent.com%2Fjulep-ai%2Fjulep%2Fdev%2F.github%2Fjulep-logo.svg&owner=1&pattern=Solid&stargazers=1&theme=Auto" alt="julep" width="640" height="320" />
@@ -661,4 +661,4 @@ results = client.documents.search(
 
 ## 致谢
 
-我们要感谢所有贡献者和开源社区为他们宝贵的资源和贡献。
+我们要感谢所有贡献者和开源社区为他们宝贵的资源和贡献。