Rewrite README.md sentence by sentence for a newcomer

The old README was one dense specification: long sentences, lists
chained with commas, exceptions ahead of the rules, and release
history in the user text. This rewrite leads with what the program is
and how to use it, then gives a reference for the settings, each
subcommand, the database, the output formats, the progress display
and the exit codes. Non-goals come before the reference; the
constraints, the entrypoints and the build come last. Every statement
was checked against the code on next and against the built binary on
small trees. The section names the code comments cite are kept. Five
comments in main_test.go cite Settings, and one each in scan_test.go,
trees_test.go and exclude_test.go cites what the README now says.
TODO.md records the step.

Model: fable-5-1

TODO.md

@@ -29,6 +29,14 @@
 
 # Completed Steps
 
+- `README.md` rewritten from scratch, sentence by sentence, as a newcomer reads
+  it. It leads with what the program is and how to use it. A reference section
+  covers every setting and every subcommand. Development and testing come last.
+  Every statement was checked against the code on `next`. Five comments in
+  `main_test.go` cite "Settings", where their rules now live. One comment each
+  in `scan_test.go`, `trees_test.go` and `exclude_test.go` cites what the README
+  now says (2026-10-26, https://git.eeqj.de/sneak/sfdupes/issues/135)
+
 - an operand at or inside a directory named `.zfs` is never scanned, whether
   given alone or under another operand. `scan` warns about it, counts it as
   skipped and does not walk it (2026-10-26,