Add VAD example for Dart API (#996)

dart-api-examples/vad/README.md

@@ -0,0 +1,21 @@
+# Introduction
+
+This example shows how to use the Dart API from sherpa-onnx for voice activity detection (VAD).
+Specifically, we use VAD to remove silences from a wave file.
+
+# Usage
+
+```bash
+dart pub get
+
+wget https://github.com/k2-fsa/sherpa-onnx/releases/download/asr-models/silero_vad.onnx
+wget https://github.com/k2-fsa/sherpa-onnx/releases/download/asr-models/lei-jun-test.wav
+
+dart run \
+  ./bin/vad.dart \
+  --silero-vad ./silero_vad.onnx \
+  --input-wav ./lei-jun-test.wav \
+  --output-wav ./lei-jun-test-no-silence.wav
+```
+
+It should generate a file `lei-jun-test-no-silence.wav`, where silences are removed.