Fixed #21986 -- Added some guidelines for database indexes.

...including:

* Indexes. This is a number one priority, *after* you have determined from

* Indexes_. This is a number one priority, *after* you have determined from

  profiling what indexes should be added. Use

  :attr:`django.db.models.Field.db_index` or

  :attr:`Field.db_index <django.db.models.Field.db_index>` or

  :attr:`Meta.index_together <django.db.models.Options.index_together>` to add

  these from Django.

  these from Django. Consider adding indexes to fields that you frequently

  query using :meth:`~django.db.models.query.QuerySet.filter()`,

  :meth:`~django.db.models.query.QuerySet.exclude()`,

  :meth:`~django.db.models.query.QuerySet.order_by()`, etc. as indexes may help

  to speed up lookups. Note that determining the best indexes is a complex

  database-dependent topic that will depend on your particular application.

  The overhead of maintaining an index may outweigh any gains in query speed.

.. _Indexes: http://en.wikipedia.org/wiki/Database_index

* Appropriate use of field types.