[gnome-boxes.wiki] Create OS Downloads (recommended downloads.xml) page

[Felipe Borges <felipeborges src gnome org>]

commit 34e55a3587a7a43e0d6af54327719c6c8a1e0ba1
Author: Felipe Borges <felipeborges gnome org>
Date:   Thu Aug 4 08:59:57 2022 +0000

    Create OS Downloads (recommended downloads.xml) page

 OS-Downloads-(recommended-downloads.xml)-page.md | 32 ++++++++++++++++++++++++
 1 file changed, 32 insertions(+)
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+The box creation Assistant has a "**Download an OS**" page which lists some freely available operating 
systems that Boxes can download. The list data is powered by 
[libosinfo](https://gitlab.gnome.org/GNOME/gnome-boxes/-/wikis/tools/libosinfo).
+
+Distributors are welcome to customize their recommended-downloads list. The default recommended-downloads 
list file is hosted at https://gnome.pages.gitlab.gnome.org/gnome-boxes-logos/recommended-downloads.xml
+
+Distros can point to their very own recommended-downloads.xml file by overriding the `org.gnome.boxes 
recommended-downloads-url` 
[GSetting](https://gitlab.gnome.org/GNOME/gnome-boxes/-/blob/main/data/org.gnome.boxes.gschema.xml).
+
+> Setting `org.gnome.boxes recommended-downloads-url` to an empty string will make Boxes use its offline 
recommended-downloads.xml file ([distributed within the source 
code](https://gitlab.gnome.org/GNOME/gnome-boxes/-/blob/main/data/recommended-downloads.xml)) instead of the 
remote address.
+
+# Overriding the recommended-downloads list
+
+Currently one can override the following properties of an OS entry in the list:
+* `<title>`: the label identifying the operating system
+* `<subtitle>`: a description and/or name of the vendor providing the operating system
+* `<url>`: an URL pointing to an ISO/image file to install the operating system
+
+## Example
+
+```xml
+<os id="http://gnome.org/gnome/nightly";>
+  <title>Custom GNOME OS Nightly Title</title>
+  <subtitle>The GNOME Project</subtitle>
+ <url>https://os.gnome.org/download/latest/gnome_os_installer.iso</url>
+</os>
+```
+
+# Proposing new entries to the recommended-downloads.xml list
+
+We encourage distributors to host their own recommended-downloads.xml file instead.
+
+If an OS entry makes sense to the overall GNOME Boxes userbase, it can be suggested by filing a Gitlab issue 
at https://gitlab.gnome.org/GNOME/gnome-boxes-logos/
+
+The new added operating system needs to have a matching entry in 
[osinfo-db](https://gitlab.com/libosinfo/osinfo-db).
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