Add Korean readme

[hw4n]

Hello, I've used kuroshiro on my most recent and favorite web project for processing Japanese to romaji so I can utilize the results when searching.

And today I thought that I should express this appreciation by translating kuroshiro's readme to Korean.

I've added README.ko-kr.md, appended link to Korean readme on 'Read this in other languages' sections.

p.s: shouldn't README.jp.md actually be named as README.ja-jp.md?

thanks again for this amazing project!