[main] Added DTD tutorial to sources

README.md

@@ -43,3 +43,4 @@ The problem in question will be demonstrated on a library application
 1. [Qt documentation for PySide6](https://doc.qt.io/qtforpython-6/PySide6/QtWidgets/)
 2. [Pythontutorial.net](https://www.pythontutorial.net/pyqt/pyqt-qtoolbar/)
 3. [Docs.sqlalchemy.org](https://docs.sqlalchemy.org/)
+4. [DTD tutorial](https://www.w3schools.com)