Revise README for etLLM Demo App (#106)

Updated the README to provide detailed instructions and information about the etLLM Demo App, including requirements, models, and build/run steps.

Author: shoumikhin

llm/apple/README.md

@@ -1 +1,60 @@
-Example iOS app that uses LLM Swift APIs in extension/llm. Priority P0
+# etLLM Demo App
+​​​
+Get hands-on with running LLMs — exported via ExecuTorch — natively on your iOS device!
+
+<table>
+  <tr>
+    <td>
+      <video src="https://github.com/user-attachments/assets/6ab7b299-cbe5-4412-bb37-cdb12738860a" muted autoplay loop playsinline></video>
+    </td>
+    <td>
+      <video src="https://github.com/user-attachments/assets/9620b7ea-1f09-460d-a120-bfbec896f486" muted autoplay loop playsinline></video>
+    </td>
+    <td>
+      <video src="https://github.com/user-attachments/assets/a9fd4af9-0ca9-4667-a56e-45c91d3e25dd" muted autoplay loop playsinline></video>
+    </td>
+    <td>
+      <video src="https://github.com/user-attachments/assets/f00d0dee-5d51-476b-9dba-70031a21089d" muted autoplay loop playsinline></video>
+    </td>
+  </tr>
+  <tr>
+    <td>
+      <video src="https://github.com/user-attachments/assets/b1981754-8660-4d6a-86fb-bf7c5010a36b" muted autoplay loop playsinline></video>
+    </td>
+    <td>
+      <video src="https://github.com/user-attachments/assets/0ad0f182-9c54-4042-9789-5e0d954e5844" muted autoplay loop playsinline></video>
+    </td>
+    <td>
+      <video src="https://github.com/user-attachments/assets/16b1bbf6-74b3-4ee9-aed7-2638a72274d7" muted autoplay loop playsinline></video>
+    </td>
+  </tr>
+</table>
+
+
+## Requirements
+- [Xcode](https://apps.apple.com/us/app/xcode/id497799835?mt=12/) 15.0 or later
+- A development provisioning profile with the [`increased-memory-limit`](https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_developer_kernel_increased-memory-limit) entitlement.
+​​​
+## Models
+​​​
+Download already exported LLMs along with tokenizers from [HuggingFace](https://huggingface.co/executorch-community) or export your own empowered by XNNPACK, Core ML or MPS backends.
+​​​
+## Build and Run
+
+1. Open the Xcode project:
+    ```bash
+    open llm/apple/etLLM.xcodeproj
+    ```
+
+2. Click the Play button to launch the app in the Simulator.
+
+3. To run on a device, ensure you have it set up for development and a provisioning profile with the `increased-memory-limit` entitlement. Update the app's bundle identifier to match your provisioning profile with the required capability.
+
+4. After successfully launching the app, copy the exported ExecuTorch model (`.pte`) and tokenizer (`.model`, `.json`, `.bin`, etc.) files to the etLLM folder. Four models are currently supported at the moment - Gemma3, LLaMa, LLaVA, Qwen3, Phi4, SmolLM3, and Voxtral. Please ensure that your model `.pte` file starts with a corresponding prefix (e.g. `llama`, `qwen3`, `phi4`, etc.) so that the app selects the correct model type.
+
+    - **For the Simulator:** Drag and drop both files onto the Simulator window and save them in the `On My iPhone > etLLM` folder.
+    - **For a Device:** Open a separate Finder window, navigate to the Files tab, drag and drop both files into the etLLM folder, and wait for the copying to finish.
+
+6. Follow the app's UI guidelines to select the model and tokenizer files from the local filesystem and issue a prompt.
+​​​
+For more details check out the [Using ExecuTorch on iOS](https://docs.pytorch.org/executorch/1.0/using-executorch-ios.html) page.