Flatpak: add install instructions

flatpak/README.md

+Run the following steps from a directory containing the gajim source dir.
+
+Install gajim flatpak repo
+--------------------------
+
+1. `flatpak --user remote-add --from gnome https://sdk.gnome.org/gnome.flatpakrepo`
+1. `flatpak --user install gnome org.gnome.Platform//3.24`
+1. `flatpak --user install gnome org.gnome.Sdk//3.24`
+1. `flatpak-builder --repo=repo directory gajim/org.gajim.Gajim.json`
+1. `flatpak --user remote-add --no-gpg-verify repo repo`
+1. `flatpak --user install repo org.gajim.Gajim`
+1. `flatpak run org.gajim.Gajim`
+
+Update gajim flatpak repo
+-------------------------
+
+1. update your gajim source repository
+1. `rm -r directory`
+1. `flatpak-builder --repo=repo directory gajim/org.gajim.Gajim.json`
+1. `flatpak --user update`
+
+Note: remove `--user` if you want a system-wide installation