[gthumb] README: added a screenshot

[Paolo Bacchilega <paobac src gnome org>]

commit cd052bef3d3fd37623ded277838bf9274e7f91bc
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Fri Nov 22 17:21:13 2019 +0100

    README: added a screenshot

 README.md | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/README.md b/README.md
index 2068e63a..a8aa7432 100644
--- a/README.md
+++ b/README.md
@@ -3,6 +3,8 @@
 An image viewer and browser utility for the GNOME environment.
 More information can be found at https://wiki.gnome.org/Apps/Gthumb.
 
+![Image](https://gitlab.gnome.org/GNOME/gthumb/raw/master/data/appdata/ss-viewer.png)
+
 ## Features
 
  * Image browser