Document install, a daily cron scan and reading the reports (closes #54)

Getting Started gains three parts: installing with go install, from a
clone or as the Docker image; a crontab line for a daily root scan,
with where its stderr and failures go; and how to read the two reports,
why a row is a candidate rather than proof, and how to check a pair
with cmp before removing anything. The usage block names --help, and
the text below it --workers and -x.

The install line uses @main, not @latest: @latest resolves to the
v0.0.1 tag, which predates the database.

Model: opus-5-5

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- README documents install, Docker, a daily cron scan and how to read
+  and check the reports (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/54)
+
 - the `Dockerfile` build stage keeps the Go module cache out of `builder`'s
   home and copies the sources with `--chown`, so no `chown -R` walks them
   (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/43)