add example description for the dart package (#1011)

+# sherpa-onnx app example
+
+## Streaming speech recognition
+
+Please see https://github.com/k2-fsa/sherpa-onnx/tree/master/dart-api-examples/streaming-asr
+
+## Non-streaming speech recognition
+
+Please see https://github.com/k2-fsa/sherpa-onnx/tree/master/dart-api-examples/non-streaming-asr
+
+## Text to speech (TTS)
+
+Please see https://github.com/k2-fsa/sherpa-onnx/tree/master/dart-api-examples/tts
+
+## Voice activity detection (VAD)
+
+Please see https://github.com/k2-fsa/sherpa-onnx/tree/master/dart-api-examples/vad
+