Simpler flatpak build instructions

README.md

@@ -15,8 +15,7 @@ Fedora and Debian packages are planned.
 
 You can also install using flatpak, which should work for every Linux distribution
 ```sh
-flatpak install org.gnome.Sdk//3.30 org.gnome.Platform//3.30
-flatpak-builder --install --user --force-clean _flatbuild ch.gnugen.Moody.json
+flatpak-builder --install-deps-from=flatpak --install --user --force-clean _flatbuild ch.gnugen.Moody.json
 ```
 For some reason, you may need to install elfutils `sudo apt install elfutils`  
 Flatpak will build libhandy and myhtml from their repositry, and use libsoup and libsecret