warning message so people know they have to use migrations for model changes

courseware/models.py

+"""
+WE'RE USING MIGRATIONS!
+
+If you make changes to this model, be sure to create an appropriate migration
+file and check it in at the same time as your model changes. To do that,
+
+1. Go to the mitx dir
+2. ./manage.py schemamigration courseware --auto description_of_your_change
+3. Add the migration file created in mitx/courseware/migrations/
+
+"""
 import uuid
 
 from django.db import models