Update README

README.md

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 # QAdwaitaDecoration
-Qt decoration plugin implementing Adwaita-like client-side decorations 
+Qt decoration plugin implementing Adwaita-like client-side decorations.
+
+## How to compile
+This library uses private Qt headers and will likely not be forward nor
+backward compatible. This library will have to be recompiled with every
+Qt update. While it can be build using Qt 5, it depends on backported
+changes from Qt 6. You can get these [here](https://src.fedoraproject.org/rpms/qt5-qtwayland/blob/rawhide/f/qtwayland-decoration-support-backports-from-qt6.patch).
+
+Build instructions:
+
+```
+mkdir build
+cd build
+cmake [OPTIONS] [-DUSE_QT6=true] ..
+make && make install
+```
+
+## Usage
+It can be used by setting the QT_WAYLAND_DECORATION environment variable:
+
+```
+export QT_WAYLAND_DECORATION=adwaita
+```
+
+## License
+The code is under [LGPL 2.1](https://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html) with the "or any later version" clause.
+