docs : add VAD model download instructions [no ci] (#3180)

Author: danbev

README.md

@@ -751,7 +751,32 @@ The following VAD models are currently supported:
 [Silero-vad](https://github.com/snakers4/silero-vad) is a lightweight VAD model
 written in Python that is fast and accurate.
 
-This model can be converted to ggml using the following command:
+Models can be downloaded by running the following command on Linux or MacOS:
+```console
+$ ./models/download-vad-model.sh silero-v5.1.2
+Downloading ggml model silero-v5.1.2 from 'https://huggingface.co/ggml-org/whisper-vad' ...
+ggml-silero-v5.1.2.bin        100%[==============================================>] 864.35K  --.-KB/s    in 0.04s
+Done! Model 'silero-v5.1.2' saved in '/path/models/ggml-silero-v5.1.2.bin'
+You can now use it like this:
+
+  $ ./build/bin/whisper-cli -vm /path/models/ggml-silero-v5.1.2.bin --vad -f samples/jfk.wav -m models/ggml-base.en.bin
+
+```
+And the following command on Windows:
+```console
+> .\models\download-vad-model.cmd silero-v5.1.2
+Downloading vad model silero-v5.1.2...
+Done! Model silero-v5.1.2 saved in C:\Users\danie\work\ai\whisper.cpp\ggml-silero-v5.1.2.bin
+You can now use it like this:
+
+C:\path\build\bin\Release\whisper-cli.exe -vm C:\path\ggml-silero-v5.1.2.bin --vad -m models/ggml-base.en.bin -f samples\jfk.wav
+
+```
+
+To see a list of all available models, run the above commands without any
+arguments.
+
+This model can be also be converted manually to ggml using the following command:
 ```console
 $ python3 -m venv venv && source venv/bin/activate
 $ (venv) pip install silero-vad