Add reference to Flatpak in preferences and persistence

In order to clarify that the principles describe in preferences and
persistence also apply to Flatpak add a small reference to it.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

content/designs/preferences-and-persistence.md

@@ -30,6 +30,10 @@ data for apps. See  [Overall architecture]( {{< ref "#overall-architecture" >}}
 The [Robustness Design]({{< ref "robustness.md" >}}) document gives more detail on the requirements for
 robustness of main storage in the face of power loss.
 
+The principles described in this document also apply to
+[The next-gen Apertis application framework]( {{< ref application-framework >}} )
+for which Apertis will use Flatpak.
+
 ## Terminology and concepts
     
 ### System Settings