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

The original and its JSON stay at the save path; the next run removes
the file's 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 and its JSON at its save path. 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").