Add 'building' section in the README, which is the same as repo 'hypr…

…land-qt-support'

README.md

@@ -5,3 +5,15 @@ This repo houses some qt/qml utilities that might be used by various hypr* apps.
 ## Dependencies
 
 This depends on qt6 and qt6-qml, as well as hyprland-qt-support.
+
+## Building
+
+You can build it with this command:
+```sh
+cmake --no-warn-unused-cli -DCMAKE_BUILD_TYPE:STRING=Release -DCMAKE_INSTALL_PREFIX:PATH=/usr -DINSTALL_QML_PREFIX=/lib/qt6/qml -S . -B ./build
+cmake --build ./build --config Release --target all -j`nproc 2>/dev/null || getconf NPROCESSORS_CONF`
+```
+
+Please note the `INSTALL_QML_PREFIX` is _distro-specific_ and may differ.
+
+For Arch, it's the same as in the example above, `/lib/qt6/qml`.