add text README.ja.md

I translated the text that has been added to README.md in this commit (https://github.com/sinatra/sinatra/commit/0cb66792c8dd057996b5bbda1b39892e33b80b94#diff-04c6e90faac2675aa89e2176d2eec7d8).

And, I have appended to README.ja.md this sentence.

I found some wasted spaces in README.ja.md, so I have modified along the way.