[preservefiles] Example with REQUIRES

- preservefiles generally needs to have the target filesystems
   mounted, so that it can preserve to them; but you can also
   configure it such that there is no need for mounted filesystems
   (e.g. in OEM setup).
 - Add an example line in CMakeLists.txt to show how that would be done.

src/modules/preservefiles/CMakeLists.txt

@@ -8,6 +8,7 @@ calamares_add_plugin( preservefiles
         PreserveFiles.cpp
     LINK_PRIVATE_LIBRARIES
         calamares
+    # REQUIRES mount  # To set the rootMountPoint
     SHARED_LIB
     EMERGENCY
 )