Add instruction how to use Japanese fonts

[mshr-h]

Clean White theme uses Chinese fonts by default. But Japanese contents usually use Japanese fonts. It looks really weird when rendering Japanese contents using Chinese font. Adding some instructions how to properly render Japanese is helpful for many Japanese users. Can I add it? If it's ok, I'll send a PR.

[zhaohuabing]

Sure, that would be great.

[stale[bot]]

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