Rewrite README.md sentence by sentence for a newcomer

The old README was a specification written as one dense document:
sentences past 30 words, lists chained with commas, exceptions ahead
of the rules, and release history in the user text. This rewrite
leads with what the program is, what it is for 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 what the built binary prints on
a small tree. The section names the code comments cite are kept, and
three comments in main_test.go now cite Settings. TODO.md records the
step.

Model: fable-5-1

main_test.go

@@ -648,7 +648,7 @@ func TestRunUsageErrors(t *testing.T) {
 }
 
 func TestRunScanRejectsWorkersBelowOne(t *testing.T) {
-	// README §scan mode: --workers below 1 is a usage error reported in
+	// README §Settings: --workers below 1 is a usage error reported in
 	// one line on stderr, before the scan opens the database.
 	for _, workers := range []string{"0", "-1"} {
 		t.Run(workers, func(t *testing.T) {
@@ -686,8 +686,8 @@ func TestRunScanRejectsWorkersBelowOne(t *testing.T) {
 func TestRunHelp(t *testing.T) {
 	t.Parallel()
 
-	// README §Subcommands: help goes to stderr, exits 0, and leaves
-	// stdout empty.
+	// README §Settings: help goes to stderr, exits 0, and leaves stdout
+	// empty.
 	cases := [][]string{{"--help"}, {"-h"}, {cmdScan, "--help"}}
 
 	for _, args := range cases {
@@ -713,8 +713,8 @@ func TestRunHelp(t *testing.T) {
 func TestRunVersion(t *testing.T) {
 	t.Parallel()
 
-	// README §Subcommands: the version is one line on stdout, with
-	// nothing on stderr, and exits 0.
+	// README §Settings: the version is one line on stdout, with nothing
+	// on stderr, and exits 0.
 	info, _ := debug.ReadBuildInfo()
 
 	for _, arg := range []string{"--version", "-v"} {