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

README.md

@@ -484,15 +484,6 @@ levels. From the quietest, the levels are fatal, error, warning, info, debug and
 trace. A run prints the lines at its own level and at every quieter one. The
 level is info unless `-v`, `-vv` or `-q` says otherwise.
 
-Two things on stderr are not log lines. A command line quak cannot parse, such
-as a missing argument, an unknown option or `-q` with `-v`, is refused with an
-`error: …` message, and quak exits 1. `-q` does not hide the message. `quak`
-with no command prints the usage on stderr and exits 1.
-
-`yarn quak` prints yarn's own lines on stdout as well. To pipe a command's
-output, run `yarn --silent quak files --json` or
-`node dist/bin/quak.js files --json` instead.
-
 - Fatal: why a command exits non-zero. A command that runs and exits non-zero
   logs at least one fatal line. A command that exits 1 because files or albums
   failed logs it last, saying how many failed and where to look.
@@ -517,6 +508,15 @@ output, run `yarn --silent quak files --json` or
 one that exits non-zero prints only its fatal lines. The prompts of `login` are
 not log lines, and `-q` leaves them alone.
 
+Two things on stderr are not log lines. A command line that quak cannot parse,
+such as a missing argument, an unknown option or `-q` with `-v`, is refused with
+an `error: …` message, and quak exits 1. `-q` does not hide the message. `quak`
+with no command prints the usage on stderr and exits 1.
+
+`yarn quak` prints yarn's own lines on stdout as well. To pipe a command's
+output, run `yarn --silent quak files --json` or
+`node dist/bin/quak.js files --json` instead.
+
 With `--json`, a command's JSON on stdout takes the place of its summary on
 stderr. For `backup`, that is the `--- Backup complete ---` line, the counts and
 the `Failed files:` list. For `list-missing-thumbnails`, it is
@@ -962,9 +962,9 @@ of the same name apart.
 `files`. Each `<fileID>.json` holds the fields a backup's JSON beside an
 original holds, with these differences:
 
-- `imageMetadata` is written only with `--exif`. The original of every file is
-  then downloaded, videos included, and read as a backup reads it (see "The JSON
-  beside each original").
+- `imageMetadata` is written only with `--exif`. The original of every file,
+  videos included, is then downloaded and read for its EXIF, XMP and JPEG
+  dimensions.
 - A file whose original holds no image metadata gets no `imageMetadata` field.
 - An original that cannot be downloaded or read gives `imageMetadataError`
   instead.