README improvements

 * Advise ``TIME_ZONE = 'UTC'`` so that times in the database don't
   require migration when moving to ``USE_TZ = True`` (otherwise you
   must dump the database, fiddle with settings.py, flush, load—praying
   that no dates fall on a DST transition—and fiddle again).
 * Fix reST syntax error.
Merge pull request #15 from twm/readme-improvements

README improvements