In README.md, Add link to Japanese translation of keras-vis document

[keisen]

We have been writing Japanese translation of keras-vis document .

• The project is https://github.com/keisen/keras-vis-docs-ja
• The document being written is https://keisen.github.io/keras-vis-docs-ja/

We are looking for reviewers who is able to understand the tech of keras-vis written in Japanese. Please help us with creating more good document.

[keisen]

@raghakot, I’m going to self-merge, but please revert if there is any problem.

[raghakot]

This is amazing work. Thanks.