Document the retagger

Fixes #54

docs/utilities.rst

@@ -105,3 +105,30 @@ import, so should you can now safely delete the entire project directly,
 database, encrypted PDFs and all, and later create it all again simply by
 running the consumer again and dumping all of these files into
 ``CONSUMPTION_DIR``.
+
+
+.. _utilities-retagger:
+
+The Re-tagger
+-------------
+
+Say you've imported a few hundred documents and now want to introduce a tag
+and apply its matching to all of the currently-imported docs.  This problem is
+common enough that there's a tool for it.
+
+
+.. _utilities-retagger-howto:
+
+How to Use It
+.............
+
+This too is done via the ``manage.py`` script:
+
+.. code:: bash
+
+    $ /path/to/paperless/src/manage.py document_retagger
+
+That's it.  It'll loop over all of the documents in your database and attempt
+to match all of your tags to them.  If one matches, it'll be applied.  And
+don't worry, you can run this as often as you like, it' won't double-tag
+a document.