[JavaFX-Part 4] Add explanation on fx:root

[JiaXinEu]

Current:

Problem:

The tutorial states to check Use fx:root construct but does not explain what it does.

Proposed:

Insert a link to the JavaFX-FXML documentation on what is fx:root and add the following:
Checking this enables the use of the fx:root element, allowing you to reference a predefined root element. More about fx:root on the documentation Introduction to FXML | JavaFX 2.2 (oracle.com).

[damithc]

@JiaXinEu You can send a PR for this.