Add README for Streamlit example

examples/python/streamlit-api/README.md

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+# Streamlit API Example
+
+This is a simple Streamlit application that demonstrates how to run Streamlit apps in .NET Aspire using the Python Extensions.
+
+## Prerequisites
+
+- Python 3.12 or later
+- .NET 8.0 or later
+- Streamlit package
+
+## Setup
+
+1. Create a virtual environment:
+   ```bash
+   python -m venv .venv
+   ```
+
+2. Activate the virtual environment:
+   - On Windows:
+     ```bash
+     .venv\Scripts\activate
+     ```
+   - On macOS/Linux:
+     ```bash
+     source .venv/bin/activate
+     ```
+
+3. Install dependencies:
+   ```bash
+   pip install streamlit
+   ```
+
+## Running the App
+
+The app can be run as part of the Aspire AppHost project or standalone.
+
+### Run with Aspire
+
+Navigate to the AppHost project and run:
+```bash
+dotnet run
+```
+
+### Run Standalone
+
+From this directory, run:
+```bash
+streamlit run app.py
+```
+
+## About the App
+
+This is a basic Streamlit app that demonstrates:
+- Simple text display
+- Environment variable reading (PORT)
+- Session state management
+- Interactive buttons