Document use of the testsite for manual testing

[PeterJCLaw]

This appears to be possible and somewhat useful, though a little non-obvious given that it doesn't have migrations already generated.

In case it's useful, the steps I used were (approximately):

createdb testsite # (ask Postgres to create a bootstrap DB for the site)
export TEST_DATABASE_NAME=testsite

tests/testsite/manage.py makemigrations photofeed polls turtles
tests/testsite/manage.py migrate

tests/testsite/manage.py shell --command '
import datetime
from polls.models import Question
Question.objects.create(question_text="Who are you?", pub_date=datetime.datetime.now())
'

tests/testsite/manage.py devdata_export

TEST_DATABASE_NAME=testsite_import tests/testsite/manage.py devdata_import