update "User's Guide"
update "User's Guide"

- add a second tutorial, based on the Rheingold zettels in zg-tutorial.
Show how to

  (1) output a list of tags used in a database.
  (2) add tags to multiple zettels with a shell loop. (adapted from tutorial-Terminal.)

- update the synopses

- remove markdown headings from the list of fields (to improve
web rendering).

ZettelGeist/zettelgeist,ZettelGeist/zettelgeist,ZettelGeist/zettelgeist