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. Comments in `main_test.go`, `scan_test.go`, `trees_test.go` and
`exclude_test.go` cite 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, no Windows excludes, 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

scan_test.go

@@ -1064,8 +1064,9 @@ func TestScanOneFilesystemWalksNestedFileOperand(t *testing.T) {
 	}
 }
 
-// buildSmokeTree recreates the README smoke-test filesystem layout
-// with deterministic content and returns the tree root.
+// buildSmokeTree recreates the tree of the test under README
+// "Definition of done" with deterministic content and returns the tree
+// root.
 func buildSmokeTree(t *testing.T) string {
 	t.Helper()