Add unique constraint, None option

docs/migration.rst

@@ -4,3 +4,16 @@ Schema Migrations
 Datalite provides a module, ``datalite.migrations`` that handles schema migrations. When a class
 definition is modified, ``datalite.migrations.basic_migration`` can be called to automatically
 transfer records to a table fitting the new definitions.
+
+Let us say we have made changes to the fields of a dataclass called ``Student`` and now,
+we want these changes to be made to the database. More specifically, we had a field called
+``studentt_id`` and realised this was a typo, we want it to be named into ``student_id``,
+and we want the values that was previously hold in this column to be persistent despite the
+name change. We can achieve this easily by:
+
+.. code-block:: python
+
+    datalite.basic_migration(Student, {'studentt_id': 'student_id'})
+
+This will make all the changes, if we had not provided the second argument,
+the values would be lost.