Add note to README

Author: synesthesiam

README.md

@@ -33,4 +33,7 @@ docker run -it -p 10300:10300 -v /path/to/local/data:/data rhasspy/wyoming-whisp
     --model tiny-int8 --language en
 ```
 
+**NOTE**: Models are downloaded temporarily to the `HF_HUB_CACHE` directory, which defaults to `~/.cache/huggingface/hub`.
+You may need to adjust this environment variable when using a read-only root filesystem (e.g., `HF_HUB_CACHE=/tmp`).
+
 [Source](https://github.com/rhasspy/wyoming-addons/tree/master/whisper)