Documentation: missing tutorial to setup sample user account for debexpo

It seems that docs/*.rst describes the process to setup debexpo local development well, but missing about creating a sample user account to start working on it.

Without it, it may causes the following error when importer is launched by manually.(celery --app debexpo call debexpo.importer.tasks.importer)

self.field.remote_field.model._meta.object_name,
ValueError: Cannot assign "'XXX'": "PackageUpload.uploader" must be a "User" instance.