Clarify about writable folders when using OSTree

Since when using OSTree folder /home is writable and preserved across
updates clarify this to avoid misunderstandings and to be consistent
with the rest of the document.

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

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

@@ -377,7 +377,8 @@ to make the system unbootable.
 - Besides the exceptions of `/var` and `/etc` directories, the rest of the
   contents of the tree are checked out as hard links into the repository.
 - Both `/etc` and `/var` are persistent writable directories that get preserved
-  across upgrades.
+  across upgrades. Additionally since `/home` is used to store user specific data
+  it is also writable and preseved across updates.
 
 ### Resilient upgrade workflow