add postgres example instructions

heromodels/examples/postgres_example/README.md

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+# PostgreSQL Model Example
+
+This example demonstrates the Hero Models framework's PostgreSQL integration capabilities, showcasing how to create, store, retrieve, and manage models in a PostgreSQL database.
+
+## Quick Setup
+
+**Automated Setup (Recommended):**
+```bash
+./setup.sh
+```
+
+The setup script will automatically:
+- Detect your operating system (macOS, Ubuntu/Debian, CentOS/RHEL)
+- Install PostgreSQL if not already installed
+- Start the PostgreSQL service
+- Create the required database user with password
+- Test the database connection
+- Configure PATH (macOS only)
+
+**Manual Setup:**
+If you prefer to set up PostgreSQL manually, the example expects:
+- PostgreSQL server running on `localhost:5432`
+- Username: `postgres`
+- Password: `test123`
+
+## What This Example Demonstrates
+
+### Core Features
+1. **Model Creation** - Creating User and Comment models with the Hero Models framework
+2. **Database Operations** - Storing, retrieving, updating, and deleting records
+3. **Indexing** - Using username and active status indexes for efficient queries
+4. **Relationships** - Associating comments with users
+5. **Connection Management** - PostgreSQL connection pooling and configuration
+
+### Specific Operations
+- **User Management**: Create users with different attributes (username, email, active status)
+- **Index Queries**: Retrieve users by username and filter by active status
+- **Data Deletion**: Remove users and see the impact on queries
+- **Comment System**: Create comments and associate them with users
+- **Model Introspection**: Display model information and database prefixes
+
+## Running the Example
+
+From the heromodels root directory:
+
+```bash
+cargo run --example postgres_model_example
+```
+
+## Expected Output
+
+The example will:
+1. Create 4 sample users with different attributes
+2. Display all users retrieved from the database
+3. Demonstrate index-based queries (by username and active status)
+4. Delete a user and show the updated results
+5. Create and associate comments with users
+6. Display model metadata information
+
+## Code Structure
+
+- **Database Configuration**: Sets up PostgreSQL connection with credentials
+- **Model Creation**: Uses fluent builder pattern for creating User and Comment instances
+- **Database Operations**: Demonstrates CRUD operations using the Hero Models API
+- **Index Usage**: Shows how to query using predefined indexes
+- **Error Handling**: Proper error handling for database operations
+
+## Key Technologies
+
+- **Hero Models Framework**: Core ORM-like functionality
+- **PostgreSQL**: Database backend with connection pooling
+- **Rust**: Type-safe model definitions and operations
+- **Serde**: JSON serialization/deserialization for database storage