Fixed #21179 -- Added a small section in the "Outputting CSV with Django" about the StreamingHttpResponse

This is my attempt to fix #21179.

The example shows how generators can be used with the csv.writer class stream large CSV files.

I have added a few comments on this example on the ticket's page: https://code.djangoproject.com/ticket/21179

This supersedes pull request https://github.com/django/django/pull/2358