Add Flutter example for speaker identification (#894)

sherpa-onnx/flutter/example/assets/README.md

@@ -0,0 +1,12 @@
+# Introduction
+
+## Speaker identification
+
+```bash
+# switch to this directory and run
+
+wget https://github.com/k2-fsa/sherpa-onnx/releases/download/speaker-recongition-models/3dspeaker_speech_eres2net_base_sv_zh-cn_3dspeaker_16k.onnx
+git clone https://github.com/csukuangfj/sr-data
+
+rm -rf sr-data/.git
+```