doc: adding readme file

Documentating the project with a README file

README.md

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+# Secure Software Design API
+
+A Spring Boot REST API demonstrating secure software design principles with token-based authentication and authorization.
+
+## Features
+
+- Token-based authentication
+- Role-based access control (RBAC)
+- BCrypt password hashing
+- HTTPS/SSL support
+- Security headers (CSP, HSTS, X-Frame-Options)
+- Input validation
+- Centralized exception handling
+
+## Prerequisites
+
+- Java 21
+- PostgreSQL
+- Maven
+
+## Configuration
+
+Create a `.env` file based on `.env.example`:
+
+```
+DB_URL=jdbc:postgresql://localhost:5432/your_database
+DB_USER=your_username
+DB_PASSWORD=your_password
+SSL_KEYSTORE_PASSWORD=your_keystore_password
+SSL_ENABLED=true
+PORT=8443
+```
+
+## Build & Run
+
+```bash
+# Build
+./mvnw clean package
+
+# Run
+./mvnw spring-boot:run
+```
+
+The API runs on `https://localhost:8443` by default.
+
+## API Endpoints
+
+### Public
+
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| POST | `/auth/login` | Authenticate user |
+| GET | `/health` | Health check |
+
+### Protected
+
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/users/me` | Get current user profile |
+| POST | `/users/create` | Create new user (Admin only) |
+| POST | `/users/logout` | Logout and invalidate tokens |
+
+## Project Structure
+
+```
+src/main/java/com/api/main/
+├── controllers/     # REST endpoints
+├── services/        # Business logic
+├── entities/        # JPA entities
+├── repositories/    # Data access
+├── security/        # Security configuration
+├── dto/             # Data transfer objects
+└── config/          # App configuration
+```