README: guard the daily cron scan against an unmounted disk (closes #175)

A disk that is not mounted leaves its mount point an empty directory.
scan then deletes every record under it, and the next scan reads every
file again. The README now says so. Its cron line runs scan only when
mountpoint finds the disks mounted, and one sentence says which mount
points to test.

Model: opus-5-5

TODO.md

@@ -29,6 +29,9 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- `README.md` says what a disk that is not mounted does to the database. The
+  daily `cron` line runs `scan` only when `mountpoint` reports `/srv` as mounted
+  (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/175)
 - The help of `sfdupes` and of `scan` names the database, its default path and
   `SFDUPES_DATABASE`. The help of `report` and `trees` lists the columns, the
   order of the rows and the escapes. The help of `info` lists its lines. The