DOCS: add documentation about how to extract FIXTURES

[agy-why]

We regularly need to extract features from running instance for development, backup... but we do not have a clear documentation on the topic.

We should add it to the developer doc.

[agy-why]

# extract examples
python manage.py dumpdata daiquiri_query.example > examples.json

# extract files
python manage.py dumpdata daiquiri_files.directory > directories.json

# extract functions
python manage.py dumpdata daiquiri_metadata.function > functions.json

# extract groups
python manage.py dumpdata auth.group > groups.json

# extract users
python manage.py dumpdata auth.user daiquiri_auth.profile account.emailaddress > users.json

[agy-why]

done in https://github.com/django-daiquiri/docs/commit/44b0035bb4619164e4e0bef940a9ad75578a97b0