Add system overview

README.md

@@ -1,5 +1,17 @@
 # Secure Notes Vault API
 
+## System overview
+
+Secure Notes Vault API is a Spring Boot REST service for creating, managing, and sharing private notes. Users register and log in with username/password credentials, then authenticate subsequent requests with a JWT bearer token.
+
+The application is organized around a standard controller-service-repository flow:
+
+- Controllers expose the authentication and notes REST endpoints.
+- Spring Security and a JWT filter authenticate protected requests.
+- Services enforce note ownership and read-only shared-note access rules.
+- JPA repositories persist users, notes, and note shares.
+- Flyway migrations manage the database schema for both local H2 and PostgreSQL-backed runs.
+
 ## Running the program
 
 This project requires Java 21 and Maven. The API starts on `http://localhost:8080`.