Update install instructions for earlier Ubuntu versions

On earlier versions of Ubuntu 21.04 some libraries (e.g.: Qt5QmlModels)
won't be deployed even requesting their installation, 
it requires to build the dependencies, 
updating the install documentation accordingly.

src/qml/README.md

@@ -77,6 +77,9 @@ they are not needed for static builds:
 ```
 sudo apt install qml-module-qtquick2 qml-module-qtquick-controls qml-module-qtquick-controls2 qml-module-qtquick-layouts qml-module-qtquick-window2 qml-module-qt-labs-settings
 ```
+##### Important:
+
+In cases where the necessary dependencies cannot be installed directly on your system, you can resort to performing a `depends build`, ensuring compatibility across different distributions. Detailed instructions can be found in the [dependencies guide](https://github.com/bitcoin-core/gui-qml/blob/main/depends/README.md).
 
 #### Fedora: