[gnome-boxes.wiki] Add link to osinfo-db database layout doc

[Felipe Borges <felipeborges src gnome org>]

commit 50ecdb1a61f20f9da0406b4ebd7fd569e102eb84
Author: Felipe Borges <felipeborges gnome org>
Date:   Thu Aug 4 15:27:44 2022 +0000

    Add link to osinfo-db database layout doc

 tools/libosinfo.md | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/tools/libosinfo.md b/tools/libosinfo.md
index eafc226..1bc31d2 100644
--- a/tools/libosinfo.md
+++ b/tools/libosinfo.md
@@ -19,6 +19,8 @@ A typical `Os` entity provides information about operating systems that is used
 
 In RPM based systems the osinfo-db is usually installed in `${_datadir}/osinfo/`. In the Boxes Flatpak, the 
osinfo-db is installed at `/app/share/osinfo-db/`.
 
+See also [The Libosinfo Database 
Layout](https://gitlab.com/libosinfo/osinfo-db-tools/-/blob/master/docs/database-layout.txt)
+
 ## osinfo-db-tools
 
 Provides utility programs to manage the Osinfo database.