Add information about OSTree and settings

Improve the system updates and rollback document with information about
OSTree and how to handle settings.

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

content/designs/system-updates-and-rollback.md

@@ -778,6 +778,36 @@ image for decompression.
 The content of the update file is extracted into the temporary directory
 and the signature is checked for the extracted commit tree.
 
+### Settings
+
+As described in
+[preferences and persistence]( {{< ref "preferences-and-persistence.md" >}} )
+there are different types of settings which should be preserved across updates.
+The setting should either be kept intact or updated to reflect new logic of the
+application.
+
+When using `OSTree`, most of the file system is read-only. Since system
+settings need write support, the `/etc` and `/var` partitions
+are configured to be read-write. This also applies to the `/home`
+partition, with it being configured as read-write so user data and
+settings can be preserved.
+
+During an `OSTree` upgrade, a new commit is applied on the `OSTree` repo,
+this provides the new content that will be used for the read-only portions of
+the rootfs, but does not modify the read-write parts.
+To handle the upgrade of system settings stored in `/etc`, a copy of its
+default values are kept in `/usr/etc` which is updated with the new commit.
+default values is kept in `/usr/etc` which is updated with the new commit.
+Thanks to this information `OSTree` can detect the files that have been changed
+and apply a 3-way merge, to update the `/etc`.
+
+This process allows to update settings to new defaults for files that were not
+modified and keep intact those that were.
+
+Applications are encouraged to handle settings adaptation to new version
+following the guidelines described in [user and user data management]( {{< ref "#user-and-user-data-management" >}} )
+and [preference and persistence]( {{< ref preferences-and-persistence.md >}} ).
+
 ### Error handling
 
 If for any reason the update process fails to complete, the update will