docs: add Japanese README and CONTRIBUTING files

[eltociear]

I created Japanese translated README.

[johnlanni]

Thanks

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.58%. Comparing base (ef31e09) to head (215ae20).
Report is 155 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1407      +/-   ##
==========================================
+ Coverage   35.91%   43.58%   +7.67%     
==========================================
  Files          69       76       +7     
  Lines       11576    12320     +744     
==========================================
+ Hits         4157     5370    +1213     
+ Misses       7104     6616     -488     
- Partials      315      334      +19     

see 69 files with indirect coverage changes