docs: update README

Author: evgeniy-scherbina

README.md

@@ -163,6 +163,10 @@ make fmt            # Format code
 make lint           # Lint code
 ```
 
+## Architecture
+
+For detailed information about how `boundary` works internally, see [ARCHITECTURE.md](ARCHITECTURE.md).
+
 ## License
 
 MIT License - see LICENSE file for details.