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

trees_test.go

@@ -15,8 +15,9 @@ import (
 const treesOpenLine = "[1/4] open: opening the database, elapsed 0s, " +
 	"remain unknown"
 
-// smokeTreeRecs mirrors the README smoke-test tree layout: /d/t1 and
-// /d/t2 are identical, /d/t3 differs from them only by one filename.
+// smokeTreeRecs mirrors the tree of the test under README "Definition
+// of done": /d/t1 and /d/t2 are identical, /d/t3 differs from them only
+// by one filename.
 func smokeTreeRecs() []scanRec {
 	f1Head, f1Tail, f1Content := testHash("f1h"), testHash("f1t"), testHash("f1c")
 	f2Head, f2Tail, f2Content := testHash("f2h"), testHash("f2t"), testHash("f2c")