examples : add "command" tool (#171)

examples/command/README.md

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+# command
+
+This is a basic Voice Assistant example that accepts voice commands from the microphone.
+More info is available in [issue #171](https://github.com/ggerganov/whisper.cpp/issues/171).
+
+```java
+# Run with default arguments and small model
+./command -m ./models/ggml-small.en.bin -t 8
+
+# On Raspberry Pi, use tiny or base models + "-ac 768" for better performance
+./bin/command -m ../models/ggml-tiny.en.bin -ac 768
+```
+
+## Building
+
+The `command` tool depends on SDL2 library to capture audio from the microphone. You can build it like this:
+
+```bash
+# Install SDL2 on Linux
+sudo apt-get install libsdl2-dev
+
+# Install SDL2 on Mac OS
+brew install sdl2
+
+make command
+```