[gtk/ebassi/gidocgen] headerbar: Add example image

[Matthias Clasen <matthiasc src gnome org>]

commit 2eea4a282fb9451c254c9262d2864bc3ef8f2b6a
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Feb 27 18:19:32 2021 -0500

    headerbar: Add example image

 gtk/gtkheaderbar.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/gtk/gtkheaderbar.c b/gtk/gtkheaderbar.c
index efa0df9b0e..547013f343 100644
--- a/gtk/gtkheaderbar.c
+++ b/gtk/gtkheaderbar.c
@@ -42,6 +42,8 @@
  *
  * `GtkHeaderBar` is a widget for creating custom title bars for windows.
  *
+ * ![An example GtkHeaderBar](headerbar.png)
+ *
  * `GtkHeaderBar` is similar to a horizontal `GtkCenterBox`. It allows
  * children to be placed at the start or the end. In addition, it allows
  * the window title to be displayed. The title will be centered with respect