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

README.md

@@ -109,9 +109,9 @@ read is skipped with a warning and loses its database record (see
     skips, and the summary line (see "Progress" and "`scan` mode"). The
     log grows with every scan; rotate it like any other.
 -   Skipped paths do not fail a scan: it still exits 0, and cron sends
-    nothing. A scan that fails, or is stopped by a signal, exits 1 with
-    the reason among the last lines of the log; `tail` prints them, and
-    cron mails them to root if the host can send mail.
+    nothing. A scan that fails, or is stopped by `SIGINT` or `SIGTERM`,
+    exits 1 with the reason among the last lines of the log; `tail`
+    prints them, and cron mails them to root if the host can send mail.
 -   A scan still running when the next one starts carries on. The new
     one fails at once, and the lines cron mails include
     `sfdupes: another scan is running (lock held on /var/lib/sfdupes/db.sqlite.lock)`.
@@ -126,8 +126,8 @@ Each row of `report` names two copies of one file, and each row of
 `trees` two copies of one directory tree (see "Report output format"
 and "Trees output format"). In a group of copies, the path that sorts
 first byte by byte is `first` and every other path is a `dupe` of it.
-`first` is not the original or the oldest copy; which copy to keep is
-your choice.
+`first` says nothing about which copy is the original or the oldest;
+which copy to keep is your choice.
 
 A row is a candidate, not proof:
 
@@ -154,10 +154,13 @@ frees nothing.
 A path holding a backslash, tab, newline or carriage return is escaped
 in the reports (see "Report output format"). Undo the escapes before
 using it. `printf '%b'` does exactly that, because every backslash in
-an escaped path starts one of the four escapes:
+an escaped path starts one of the four escapes. Command substitution
+drops trailing newlines, so print an `x` after the path and remove it
+afterwards, or a path that ends in a newline names a different file:
 
 ```sh
-cmp "$(printf '%b' '/srv/a/tab\tname.txt')" /srv/b/tab-copy.txt
+p="$(printf '%bx' '/srv/a/tab\tname.txt')"; p="${p%x}"
+cmp "$p" /srv/b/tab-copy.txt
 ```
 
 Check a `trees` row with `diff -r`, which compares the two trees file