document known data migration issue

refs #164

docs/source/getting-started.rst

@@ -286,6 +286,14 @@ responsibility.
 If you are using a migration library like ``alembic``, you can autogenerate a migration
 after upgrading Keg-Auth to ensure any model updates from mixins are included.
 
+__Note__: autogenerated migrations solve most of the problems, but if you are starting with
+an existing database that already has user records, you may have some data issues to resolve
+as well. The following are known issues:
+
+- Email field is expected to have all lowercase data. The model type assumes that because email
+addresses are not case-sensitive, it can coerce input to lowercase for comparison, and expects
+that persisted data matches that assumption.
+
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