README.md: CoderDojo Japan の GitHub リンクを追加

Author: yasulab

README.md

@@ -146,6 +146,9 @@ $ bundle exec rake test
 - [dojocon-japan/dojocon2017.coderdojo.jp](https://github.com/dojocon-japan/dojocon2017.coderdojo.jp)
 - [dojocon-japan/dojocon2016.coderdojo.jp](https://github.com/dojocon-japan/dojocon2016.coderdojo.jp)
 
+:yin_yang: [CoderDojo Japan - GitHub](https://github.com/coderdojo-japan) (approved by [GitHub for Nonprofit](https://news.coderdojo.jp/2019/08/29/github-for-nonprofit/))
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