Add "Qt6 QML Book" by Johan Thelin, Jürgen Bocklage-Ryannel, Cyril Lorquet (HTML, PDF) (EbookFoundation#7033)

* Add Qt6 QML Book

* Update books/free-programming-books-langs.md

Co-authored-by: David Ordás <[email protected]>

* Add PDF clarification (not link)

Co-authored-by: David Ordás <[email protected]>

Author: KotoWhiskas

books/free-programming-books-langs.md

@@ -2005,6 +2005,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 ### QML
 
 * [Qt5 Cadaques](http://qmlbook.github.io) - Juergen Bocklage-Ryannel and Johan Thelin (HTML, PDF, ePub) (:construction: *in process*)
+* [Qt6 Book](https://www.qt.io/product/qt6/qml-book/preface-preface) - Johan Thelin, Jürgen Bocklage-Ryannel, Cyril Lorquet (HTML, PDF) (:construction: *in process*)
 
 
 ### R