docs don't mention that docusign_template_id is required

[smcoll]

i see the docusign_template_id exists on the SignatureType model in the demo, but the backend itself references this property that won't exist on an existing model unless explicitly added. This results in an AttributeError. Perhaps the docs ought to mention this, or perhaps the backend should handle cases where it doesn't exist, which seems sufficient given that the model field in the demo accepts empty strings.