Flesh out description/summary

This one's purely YMMV. The description seemed a little spare,
and didn't really fit the recommendations[1], e.g:
* Include 2-3 paragraphs of interesting easy to read prose.
* Ensure you provide a description of what the application actually does.
* Describe any important features.

So I borrowed ours, which was cribbed from the `.desktop` file.

[1]: https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#sect-Quickstart-DesktopApps

xdg/openshot-qt.appdata.xml

@@ -6,11 +6,29 @@
   <project_license>GPL-3.0-or-later</project_license>
   <name>OpenShot Video Editor</name>
   <developer_name>OpenShot Studios, LLC</developer_name>
-  <summary>Video Editor</summary>
+  <summmary>
+    An easy to use, quick to learn, and surprisingly powerful video editor
+  </summary>
   <description>
     <p>
-      We designed OpenShot Video Editor to be an easy to use,
-      quick to learn, and surprisingly powerful video editor.
+      OpenShot Video Editor is a free, open-source, non-linear video editor. It
+      can create and edit videos and movies using many popular video, audio,
+      image formats. Create videos for YouTube, Flickr, Vimeo, Metacafe, iPod,
+      Xbox, and more!
+    </p>
+    <p>
+      Features include:
+      <ul>
+      <li>Multiple tracks (layers)</li>
+      <li>Compositing, image overlays, and watermarks</li>
+      <li>Audio mixing and editing</li>
+      <li>Support for image sequences (rotoscoping)</li>
+      <li>Key-frame animation</li>
+      <li>Video effects (chroma-key)</li>
+      <li>Transitions (lumas and masks)</li>
+      <li>Titles with integrated editor and templates</li>
+      <li>3D animation (titles and effects)</li>
+      </ul>
     </p>
   </description>
   <provides>