README: what a backup does with a file deleted in Ente (closes #242)

A file deleted in Ente keeps its original at its save path, and the JSON
beside it. The next run removes its symlinks from the collection
directories.

Model: opus-5-5

README.md

@@ -671,6 +671,9 @@ but letters and digits, or when it is longer than 16 characters.
 When the date or the time zone changes, the next run stores the original at its
 new path and leaves the old copy where it is.
 
+A file deleted in Ente keeps its original at its save path, and the JSON beside
+it. The next run removes its symlinks from the collection directories.
+
 The library gives the same path as `photo.savePath` and writes it with
 `photo.download()` (see "A photo's content").