README.md: Document qt 6 support.

Author: textshell

README.md

@@ -34,10 +34,14 @@ Often using signals is not the best way to achieve robust code. If possible cons
 
 ## Building / Installing
 
-    $ meson setup -Dprefix=$HOME/opt/signalmanager/ _build
+This library supports Qt 5 and Qt 6, you need to choose the qt version at compile time via the -Dqt option to meson.
+
+    $ meson setup -Dprefix=$HOME/opt/signalmanager/ -Dqt=qt6 _build
     $ ninja -C _build
     $ ninja -C _build install
 
+The library will install pkg-config .pc files for PosixSignalManager (qt 5.x version) or PosixSignalManagerQt6.
+
 ## Examples
 
 ### Reload configuration on SIGHUP