appstream: add screenshot references

Strictly speaking the <video/>-references are invalid as the referenced
file must be MKV or WebM, but neither Flathub nor `appstream-util`
have complained about it.

Some hints were provided by ovari:
https://git.jami.net/savoirfairelinux/ring-client-gnome/issues/1117#note_16987

Change-Id: I4db1c85d92c1d0d1f75eb571c3a00358c44d0def

jami-gnome.appdata.xml

@@ -25,6 +25,35 @@
  <url type="homepage">https://jami.net/</url>
  <url type="bugtracker">https://git.jami.net/savoirfairelinux/ring-client-gnome/issues/</url>
  <url type="translate">https://www.transifex.com/savoirfairelinux/jami</url>
+ 
+ <!-- Maximum caption length is 60 characters -->
+ <!-- Officially GIF is not an allowed video format, but it appears to work nonetheless -->
+ <screenshots>
+  <screenshot type="default">
+   <caption>Send Audio, Video and Chat messages</caption>
+   <image type="source" width="1310" height="650">https://dl.jami.net/media-resources/screenshots/jami_linux_audiovideo.png</image>
+  </screenshot>
+  <screenshot>
+   <caption>Easily share desktop contents</caption>
+   <image type="source" width="1016" height="659">https://dl.jami.net/media-resources/screenshots/jami_linux_screenshare.png</image>
+  </screenshot>
+  <screenshot>
+   <caption>Crystal clear audio calls between Jami users</caption>
+   <video container="webm" codec="vp9" width="600" height="400">https://dl.jami.net/media-resources/gifs_features/conversiongif_webm/audio-call_web.webm</video>
+  </screenshot>
+  <screenshot>
+   <caption>Conference calls with an unlimited participants</caption>
+   <video container="webm" codec="vp9" width="600" height="400">https://dl.jami.net/media-resources/gifs_features/conversiongif_webm/conference_web.webm</video>
+  </screenshot>
+  <screenshot>
+   <caption>Encrypted and secure text messaging, no servers</caption>
+   <video container="webm" codec="vp9" width="600" height="400">https://dl.jami.net/media-resources/gifs_features/conversiongif_webm/chat_web.webm</video>
+  </screenshot>
+  <screenshot>
+   <caption>Send files of all sizes</caption>
+   <video container="webm" codec="vp9" width="600" height="400">https://dl.jami.net/media-resources/gifs_features/conversiongif_webm/files-share_web.webm</video>
+  </screenshot>
+ </screenshots>
 
  <launchable type="desktop-id">jami-gnome.desktop</launchable>