Document migration from Weblate 2.2 to 2.3

When migrating from Django 1.6 (with South) to 1.7 (with native
migrations), some migrations have to be faked as they have been already
applies.

Fixes #770
Signed-off-by: Michal Čihař <michal@cihar.com>

docs/admin/upgrade.rst

@@ -240,6 +240,15 @@ If you have not yet performed upgrade to Django 1.7 and newer, first upgrade to
 2.2 following instructions above. Weblate 2.3 no longer supports migration from
 Django 1.6.
 
+If you were using Weblate 2.2 with Django 1.6, you will now need to fake some
+migrations:
+
+.. code-block:: sh
+
+    ./manage.py migrate --fake accounts 0004_auto_20150108_1424
+    ./manage.py migrate --fake lang 0001_initial
+    ./manage.py migrate --fake trans 0018_auto_20150213_1447
+
 Previous Weblate releases contained bug which made some monolingual
 translations behave inconsistently for fuzzy and not translated strings, if you
 have such, it is recommended to run: