Add translation into the official repository

[dalance]

This is only suggestion.
I'm a contributer of Japanese translation of RBE, and worked to merge Japanese translation into the official repository.
I think merging into the official has some advantages than the independent repository.

• [As translator] Fixes in the official will be reflected immediately
• [As translator] Tracking the official and searching the changes are not required
• [As user] The official URL (https://doc.rust-lang.org/rust-by-example/) can be used
• [As user] Through language selector in the page, language can be changed like from your language to English and vise versa

If you are interesting in this suggestion, please see https://github.com/rust-lang/rust-by-example/blob/master/TRANSLATING.md.
I can support you, if you have any question.