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

The old text served as the list of what a reader needs covered. 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 places where
the old text disagreed with it are listed in the pull request. No sentence
runs past about 30 words, every table is now a list, and no issue links or
project history remain in text meant for users. Two code comments that
named README sections now name the new ones.

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`