Move cn to zh-CN for future traditional Chinese translations

ref #2649

README.md

@@ -48,7 +48,7 @@ contains documents describing how to build and contribute to Electron.
 - [Korean](https://github.com/atom/electron/tree/master/docs-translations/ko)
 - [Japanese](https://github.com/atom/electron/tree/master/docs-translations/jp)
 - [Spanish](https://github.com/atom/electron/tree/master/docs-translations/es)
-- [Chinese](https://github.com/atom/electron/tree/master/docs-translations/cn)
+- [Simplified Chinese](https://github.com/atom/electron/tree/master/docs-translations/zh-CN)
 
 ## Community