add Usage

runtime/android/README.md

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+# Usage
+
+Most of AI engineers are not familiar with Android development, this is a simple ‘how to’.
+
+1. Train your model with your data
+
+2. Export pytorch model to onnx model
+
+3. Convert onnx model for mobile deployment
+
+```bash
+python -m onnxruntime.tools.convert_onnx_models_to_ort your-model.onnx
+```
+you will get `your-model.ort` and `your-model.with_runtime_opt.ort`
+
+
+4. Install Android Studio and open path of wekws/runtime/android and build
+
+*NOTE:* The default feature_dim in code is 40, if your model’s is 80, change it here `./app/src/main/cpp/wekws.cc`
+
+```C++
+  feature_config = std::make_shared<wenet::FeaturePipelineConfig>(40, 16000);  // 40 -> 80
+```
+
+It’s also can be built on Linux by runing `bash ./gradlew build`
+
+5. Install `app/build/outputs/apk/debug/app-debug.apk` to your phone and try it.