Updated docs
Closes: https://gitlab.com/mailman/postorius/issues/138 Standalone git repo removed mailmanclient name updated from mailman.client Virtualenv setup instructions added.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Closes: https://gitlab.com/mailman/postorius/issues/138 Standalone git repo removed mailmanclient name updated from mailman.client Virtualenv setup instructions added.