[gnome-boxes.wiki] Add testing instructions

[Felipe Borges <felipeborges src gnome org>]

commit a38f878955f3e41ba7c06969b1308272d0c01347
Author: Felipe Borges <felipeborges gnome org>
Date:   Thu Aug 4 09:06:52 2022 +0000

    Add testing instructions

 recommended-downloads.xml.md | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/recommended-downloads.xml.md b/recommended-downloads.xml.md
index a9ec0b1..ab4bcfa 100644
--- a/recommended-downloads.xml.md
+++ b/recommended-downloads.xml.md
@@ -25,6 +25,14 @@ Currently one can override the following properties of an OS entry in the list:
 </os>
 ```
 
+### Testing the changes locally
+
+You can run a simple Python http server hosting your recommended-downloads.xml file with
+
+`python -m  http.server 8000`
+
+And use `gsetting set org.gnome.boxes recommended-downloads-url <url-to-recommended-downloads.xml>` to set 
the new address from where Boxes will load its recommended downloads. 
+
 ## Proposing new entries to the recommended-downloads.xml list
 
 We encourage distributors to host their own recommended-downloads.xml file instead.