Updating install doc for Debian-based distros

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, this could also
happen with other Debian-based distros so we need to
update the install documentation accordingly.

src/qml/README.md

@@ -76,6 +76,9 @@ 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:
+
+If you're unable to install the dependencies through your system's package manager, you can instead perform a [depends build](https://github.com/bitcoin-core/gui-qml/blob/main/depends/README.md).
 
 #### Fedora: