Add comment.

model_logging/migrations/0001_initial.py

@@ -3,6 +3,11 @@
 from django.db import models, migrations
 from unittest.mock import Mock
 
+"""
+The assumption is that a user who is upgrading from 0.1.0 to this version will already have pgcrypto installed. We also check this in 0003 so that they have a verbose error message.
+
+A new user shouldn't migrate through 0001 and should go through the squashed migrations. Hence we pretend that we have a pgcrypto object to allow the migration parsing at startup to be carried out.
+"""
 try:
     import pgcrypto.fields
 except ImportError: