conventions: Add appstream command used to compose metadata

Author: bbhtt

docs/conventions.rst

@@ -178,6 +178,21 @@ The distribution usually appends those paths to ``$XDG_DATA_DIRS`` on
 the host when installing the ``flatpak`` package. Unless a desktop file is
 exported by Flatpak, host applications cannot access it.
 
+
+The metainfo file along with the other metadata such as icon and desktop
+files is composed into a catalogue by ``appstream``. Since 1.3.4,
+Flatpak Builder by default uses ``appstreamcli`` from `libappstream <https://github.com/ximion/appstream/>`_
+to compose this and executes the following command::
+
+  $ appstreamcli compose --no-partial-urls --prefix=/ --origin=${FLATPAK_ID} --media-baseurl=<repo-media-url> --media-dir=${FLATPAK_DEST}/share/app-info/media --result-root=${FLATPAK_DEST} --data-dir=${FLATPAK_DEST}/share/app-info/xmls --icons-dir=${FLATPAK_DEST}/share/app-info/icons/flatpak --components=${FLATPAK_ID} ${FLATPAK_DEST}
+
+
+``${FLATPAK_ID}`` is the Flatpak application ID and ``${FLATPAK_DEST}``
+is set to ``/app`` for apps and ``/usr`` for runtimes. The ``media-baseurl``
+controls the base URL used for mirroring screenshots. Usually each app
+store will have their own base URL for this. On Flathub this is
+set to ``https://dl.flathub.org/media/``.
+
 Technical conventions
 ---------------------