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.

Where the old text and the repository disagreed, the new text states
what the code does. #140 lists
every such point. Among them: `@latest` installs the newest tagged
release, ten million files in both "Rationale" and "Design", no Windows
excludes, a relative operand is joined to the working directory as `pwd`
prints it, and an operand reached through a symlinked directory below
another operand is dropped (#144).

Dropped from "Constraints": the permission to add `github.com/spf13/viper`
if configuration-file support is ever needed.

Model: fable-5-1

exclude_test.go

@@ -17,7 +17,9 @@ import (
 func TestDefaultExcludes(t *testing.T) {
 	t.Parallel()
 
-	// README "Rules for the walk": each system's list.
+	// README "Rules for the walk": the Linux and macOS lists, and none
+	// on any other system. The Windows list exists in the code only: the
+	// binary does not build for Windows, so the README leaves it out.
 	cases := map[string][]string{
 		goosLinux: {"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp"},
 		goosDarwin: {