Rewrite README.md sentence by sentence for a newcomer

Rewrites `README.md` from scratch for
#135. Every statement was checked
against the code on `next` and against what the built binary prints on
small trees under `/var/tmp`. The headings that the code comments cite
are kept. Five `main_test.go` comments cite "Settings", two cite what
"Error handling" says about a failed write to stdout, and one comment
each in `scan_test.go`, `trees_test.go` and `exclude_test.go` cites what
the README now says.

Where the old text and the repository disagreed, the new text states
what the code does: `@latest` installs the newest tagged release, ten
million files in both "Rationale" and "Design", no Windows excludes, and
an operand reached through a symlinked directory below another operand
is dropped (#144).

Model: fable-5-1

README.md

@@ -365,7 +365,7 @@ default. An operand may be a directory or a regular file.
 - No other case keeps an operand under another. An operand reached through a
   symlinked directory below the outer operand is dropped too, without a line.
   The outer operand's walk does not follow the symlink, so no file under the
-  inner operand is scanned. The records an earlier scan left under it are
+  inner operand is scanned. The records that an earlier scan left under it are
   deleted.
 - An operand that is neither a regular file nor a directory is not scanned.
   `scan` prints a warning naming the path and what it is. It counts the operand
@@ -466,8 +466,8 @@ walk practical on large or busy storage pools.
     operand at the path given, even when that path runs through a symlinked
     directory. Such an operand can lead to the database files, and the walk
     passes over them there as well. The walk prints no line for the database
-    files and does not count them as skipped. A record an earlier scan left for
-    one of them is deleted.
+    files and does not count them as skipped. A record that an earlier scan left
+    for one of them is deleted.
 
 - By default the walk enters directories on other filesystems than its
   operand's. With `-x`, it never enters a directory on a different filesystem
@@ -556,7 +556,8 @@ walk practical on large or busy storage pools.
 - A file or directory that disappears while the scan runs is not an error. The
   scan prints no line for it and does not count it as skipped. A path gone when
   the walk reaches it gets no record, and so does a file gone when the hash
-  phase opens it. The records an earlier scan left at or below it are deleted.
+  phase opens it. The records that an earlier scan left at or below it are
+  deleted.
 
 #### Concurrency
 
@@ -668,8 +669,8 @@ Four bytes are written as escapes:
 
 Every other byte is written unchanged, including bytes that are not valid UTF-8,
 the usual text encoding. Undoing the four escapes gives back the stored path.
-Grouping and ordering use the stored path. The warnings that `scan` prints are
-escaped the same way, so each warning stays one line.
+Grouping and ordering use the stored path. The warnings about skipped paths that
+`scan` prints are escaped the same way, so each stays one line.
 
 The summary line on stderr gives:
 
@@ -1037,9 +1038,9 @@ On a terminal:
     - the elapsed time;
     - the time remaining.
 
-    On a terminal narrower than about 75 columns, a line is cut short only hours
-    into a scan of millions of files. A terminal made narrower during a phase is
-    fitted at the next draw.
+    On a terminal 80 columns wide, no line is cut short, even hours into a scan
+    of millions of files. A terminal made narrower during a phase is fitted at
+    the next draw.
 
 - A warning printed during a phase lands on a line of its own, never inside the
   display. So does every line written to stdout when stdout and stderr are the

main_test.go

@@ -773,8 +773,8 @@ func TestBuildVersion(t *testing.T) {
 func TestRunVersionWriteFailureIsFatal(t *testing.T) {
 	t.Parallel()
 
-	// README §Error handling: a stdout write failure exits 1, reported
-	// in one line on stderr.
+	// README §Error handling: a failed write to stdout is a fatal error,
+	// exit 1, reported on stderr starting with "sfdupes:".
 	var stderr bytes.Buffer
 
 	code := run([]string{"--version"}, failingWriter{}, &stderr)
@@ -1445,8 +1445,8 @@ func TestRunReportsDuringScan(t *testing.T) {
 }
 
 func TestRunStdoutClosedIsFatal(t *testing.T) {
-	// README §Error handling: a stdout write failure exits 1, reported
-	// in one line on stderr.
+	// README §Error handling: a failed write to stdout is a fatal error,
+	// exit 1, reported on stderr starting with "sfdupes:".
 	for _, name := range []string{cmdReport, cmdTrees, cmdInfo} {
 		t.Run(name, func(t *testing.T) {
 			t.Setenv(databaseEnv, testDBPath(t))