Add Dart binding (#869)

Author: lindeer

README.md

@@ -118,6 +118,7 @@ Document 文檔: https://byvoid.github.io/OpenCC/
 * WebAssembly: [wasm-opencc](https://github.com/oyyd/wasm-opencc)
 * Browser Extension: [opencc-extension](https://github.com/tnychn/opencc-extension)
 * Go (Pure): [OpenCC for Go](https://github.com/longbridgeapp/opencc)
+* Dart (native-assets): [opencc-dart](https://github.com/lindeer/opencc-dart)
 
 ### Configurations 配置文件