Merge #353: Update install instructions for Debian-based distros when dependencies are unable to be installed

d02a23c Updating install doc for Debian-based distros (pablomartin4btc)

Pull request description:

  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.

  I've got this warning during the QML building on Ubuntu 20.04 and ofc the bitcoin-qt executable wasn't available when it finished:
  `configure: WARNING: Qt5QmlModels >= 5.11.3 not found; bitcoin-qt frontend will not be built`

  hebasto helped me to find the solution and after performing the above, managed to get it installed, up and running.

ACKs for top commit:
  hebasto:
    re-ACK d02a23c.

Tree-SHA512: fbbbeff9f5a5b2709b9939eb579a2f1b6f1f196de384598fd2fdc5d99c6e212ef67a6989244ed91dc06e8b3da119a0bd065551a589a44901218ea1dc3cf5adad

Author: hebasto

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](../../depends/README.md).
 
 #### Fedora: