[gnome-boxes.wiki] Update welcome tutorial

[Felipe Borges <felipeborges src gnome org>]

commit 74d739d321ab05e3116d6bf82989a25d50ba972d
Author: Felipe Borges <felipeborges gnome org>
Date:   Tue Dec 10 14:12:36 2019 +0000

    Update welcome tutorial

 welcome-tutorial.md | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)
---
diff --git a/welcome-tutorial.md b/welcome-tutorial.md
index 6d3f665..d6a63b0 100644
--- a/welcome-tutorial.md
+++ b/welcome-tutorial.md
@@ -1 +1,17 @@
-WIP
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+# "Welcome Tutorial" dialog
+
+The "Welcome Tutorial" dialog appears the first time a user runs GNOME Boxes in their system. The underlying 
setting that triggers this can be set with:
+
+`gsettings set org.gnome.boxes first-run true`
+
+## Adding new pages to the Tutorial
+
+The 
[data/ui/welcome-tutorial.ui](https://gitlab.gnome.org/GNOME/gnome-boxes/tree/master/data/ui/welcome-tutorial.ui)
 file holds the content definitions for the dialog.
+
+Each `GtkStack` child is a page. The page order is respected.
+
+A page is an object of type 
[Boxes.WelcomeTutorialPage](https://gitlab.gnome.org/GNOME/gnome-boxes/tree/master/data/ui/welcome-tutorial-page.ui).
 It has the following properties:
+* **title**: a string
+* **description**: a string
+* **color**: a RGB code for the background color
+* **image**: the resource path of the image file illustrating the page
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