Fixed #981 -- Documented backend-specific SQL files

``reset`` commands in ``manage.py``. Refer to the `manage.py documentation`_

for more information.

Note that if you have multiple SQL data files, there's no guarantee of the

order in which they're executed. The only thing you can assume is that, by the

time your custom data files are executed, all the database tables already will

have been created.

.. _`manage.py documentation`: http://www.djangoproject.com/documentation/django_admin/#sqlinitialdata-appname-appname

Database-backend-specific SQL data

----------------------------------

There's also a hook for backend-specific SQL data. For example, you can have

separate initial-data files for PostgreSQL and MySQL. For each app, Django

looks for a file called ``<appname>/sql/<modelname>.<backend>.sql``, where

``<appname>`` is your app directory, ``<modelname>`` is the model's name in

lowercase and ``<backend>`` is the value of ``DATABASE_ENGINE`` in your

settings file (e.g., ``postgresql``, ``mysql``).

Backend-specific SQL data is executed before non-backend-specific SQL data. For

example, if your app contains the files ``sql/person.sql`` and

``sql/person.postgresql.sql`` and you're installing the app on PostgreSQL,

Django will execute the contents of ``sql/person.postgresql.sql`` first, then

``sql/person.sql``.