Add README.ru-RU.md and CONTRIBUTING.ru-RU.md following the existing
localization pattern (zh-CN, ja-JP, ko-KR), and add the Русский link to
the language switcher line in all README and CONTRIBUTING variants.
Also add the previously missing 한국어 link to the CONTRIBUTING.zh-CN.md
and CONTRIBUTING.ja-JP.md switchers.
Closes#100
