docs: Add model downloading instructions to README

This commit adds a crucial "Downloading Models" section to the README.md.

- Explains that models are not included in the Docker image and must be downloaded manually.
- Provides detailed instructions for downloading Piper models from Hugging Face, including an example for the  voice.
- Clarifies the expected directory structure for placing the downloaded models.
- Mentions that StyleTTS is currently a placeholder and does not require model downloads.

README.md

@@ -20,6 +20,43 @@ This guide covers local development. For information on using the container regi
 - [Docker](https://docs.docker.com/get-docker/)
 - [Docker Compose](https://docs.docker.com/compose/install/)
 
+### Downloading Models (Crucial Step!)
+
+The Docker image for AudioEngineHub does *not* include the large TTS model files to keep the image small and portable. You need to **manually download** the models for the engines you wish to use and place them in the correct local directory. The `docker-compose.yml` then makes these models available to the container via a volume mount.
+
+#### Piper Models
+
+*   **Source:** [https://huggingface.co/rhasspy/piper-voices/tree/main](https://huggingface.co/rhasspy/piper-voices/tree/main)
+
+**Instructions:**
+
+1.  Go to the link above and navigate to a voice you want to use (e.g., `en/en_GB/vctk/medium/`).
+2.  For each voice, you need to download two files:
+    *   The `.onnx` model file (e.g., `en_GB-vctk-medium.onnx`)
+    *   The corresponding `.onnx.json` configuration file (e.g., `en_GB-vctk-medium.onnx.json`)
+3.  Create a directory for the voice inside your local `app/models/piper/` directory. The directory name must match the model's base name (e.g., `en_GB-vctk-medium`).
+4.  Place both downloaded files into that new directory.
+
+**Example: Setting up `en_GB-vctk-medium`:**
+
+Your local directory structure should look like this:
+```
+AudioEngineHub/
+├── app/
+│   ├── models/
+│   │   ├── piper/
+│   │   │   ├── en_GB-vctk-medium/  <-- This directory's name MUST match the model name
+│   │   │   │   ├── en_GB-vctk-medium.onnx
+│   │   │   │   └── en_GB-vctk-medium.onnx.json
+│   │   └── styletts/ # Placeholder, no external models currently needed
+│   └── ...
+├── ...
+```
+
+#### StyleTTS Models
+
+The `styletts` engine is currently a placeholder (dummy implementation) and does not require external model downloads at this time. Its `list_models()` method provides hardcoded model names.
+
 ### Local Development Setup
 
 1.  **Clone the repository:**
@@ -33,7 +70,7 @@ This guide covers local development. For information on using the container regi
     ```bash
     cp .env.example .env
     ```
-    Open the `.env` file and configure the `ACTIVE_ENGINES` list to include the engines you want to use. For example:
+    Open the `.env` file and configure the `ACTIVE_ENGINES` list to include the engines you want to use. Make sure the model directories exist for activated engines (e.g., if you enable `piper`, ensure its models are downloaded). For example:
     ```
     ACTIVE_ENGINES='["piper", "styletts"]'
     ```