Add Flutter example for speaker identification (#894)

sherpa-onnx/flutter/notes.md

@@ -0,0 +1,42 @@
+# Usage
+
+## macOS
+
+1. Build required libraries
+
+```bash
+git clone https://github.com/k2-fsa/sherpa-onnx
+cd sherpa-onnx
+mkdir build
+cd build
+
+cmake -DCMAKE_INSTALL_PREFIX=./install -DBUILD_SHARED_LIBS=ON -DCMAKE_OSX_ARCHITECTURES="x86_64;arm64" ..
+make install
+cd ../sherpa-onnx/flutter/
+cp -v  ../../build/install/lib/lib* ./macos/
+```
+
+2. Test for speaker identification
+
+```bash
+cd sherpa-onnx/sherpa-onnx/flutter/example
+mkdir assets
+```
+
+
+## Useful commands
+```
+flutter pub publish --dry-run
+flutter run -d macos
+flutter run -d linux
+flutter run -d windows
+
+flutter build macos
+
+flutter run --release -d macos
+
+# add platform to an existing project
+flutter create --platforms=windows,macos,linux .
+
+dart analyze
+```