add test build command to readme

README.md

@@ -46,6 +46,14 @@ find_package( fggl REQUIRED PATHS /tmp/fggl-lib )
 target_link_libraries(game fggl::fggl)
 ```
 
+### Flatpak Demo
+
+This is still experimental, but the command used to test flatpak compatablity with the demo application is:
+
+```
+flatpak-builder builds/flatpak build-aux/com.fossgalaxy.fggl.demo.yml --force-clean --install --user
+```
+
 ## Useful Tools
 The following tools are helpful for development: