README.md rewritten sentence by sentence (closes #219)

The old text served as the list of what to cover. The new text leads
with what quak is, what it is for and how to use it. Every command,
option, exit code and file layout follows in reference sections, then the
library reference, then design, entrypoints and development. Every
statement was checked against the code on next; the pull request lists
each disagreement. Sentences stay under about 30 words, tables are lists,
and no issue links or project history remain in text for users. Two code
comments now name the new README sections, and the one in
`src/cli-log.ts` matches "What quak prints" on stderr, stdout and exit
status; a third no longer cites the README.

Model: fable-5-1

test/cli/backup-end-to-end.test.ts

@@ -418,7 +418,7 @@ describe("the first backup of the account", () => {
         expect(downloads.sort()).toEqual([100, 200, 300]);
     });
 
-    // The tree "Backup layout" in the README describes. Each original is at
+    // The tree under "The backup directory" in the README. Each original is at
     // its save path, `YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.<fileID>.<ext>`, with
     // the extension of its title; the live photo is its image and its video,
     // each with the extension it had inside the ZIP, and a `.livephoto.json`