Add README-CN.md

Would you consider adding a Chinese version to the README.md , this will make it easier for Chinese developers to participate in the project, if possible, I can do this work.

Start by reading README.md in the grpc-java repository and check whether any existing translated README conventions are present. Done means adding README-CN.md as a Chinese version of the README, with the translation covering the same project information.

索引モデルが issue の本文から書いたものです。