[gnome-user-docs] Note about installing gnome-sushi

[Petr Kovář <pmkovar src gnome org>]

commit f7bcf724370e237af74f9bdb2839fea30dd25063
Author: Gunnar Hjalmarsson <gunnarhj ubuntu com>
Date:   Fri Mar 9 23:02:55 2018 +0100

    Note about installing gnome-sushi
    
    https://bugzilla.gnome.org/show_bug.cgi?id=794187

 gnome-help/C/files-preview.page |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)
---
diff --git a/gnome-help/C/files-preview.page b/gnome-help/C/files-preview.page
index 3875d21..2a6d06b 100644
--- a/gnome-help/C/files-preview.page
+++ b/gnome-help/C/files-preview.page
@@ -1,6 +1,7 @@
 <page xmlns="http://projectmallard.org/1.0/";
       xmlns:its="http://www.w3.org/2005/11/its";
-      type="topic" style="task"
+      xmlns:if="http://projectmallard.org/if/1.0/";
+      type="topic" style="task" version="1.0 if/1.0"
       id="files-preview">
 
   <info>
@@ -28,6 +29,13 @@
 
 <title>Preview files and folders</title>
 
+<note if:test="platform:ubuntu" style="important">
+  <p>You need to have <app>Sushi</app> installed on your computer to perform
+  these steps.</p>
+  <p><link style="button" href="apt:gnome-sushi">Install
+  <app>Sushi</app></link></p>
+</note>
+
 <p>You can quickly preview files without opening them in a full-blown
 application.  Select any file and press the space bar. The file will open in a
 simple preview window. Press the space bar again to dismiss the preview.</p>