Updated docs

docs/migrations.md

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+# Migrations
+
+Adding or removing a field and other such simple operations are automatic, but when the time comes to really change your data model, implement a `migrate_v1` function that converts your old data to the new form. This works on plain built-in dict and other types, to avoid needing to preserve old versions of your structs.
+
+Pass a module (or import path) containing `migrate_vN` functions:
+
+```python
+kanta = Kanta("data.kantadb", Data(), migrations="myapp.migrations")
+await kanta.open()
+```
+
+Kanta tracks migration version metadata automatically, and fast forwards your database to current version by running all the migrations needed while opening the database.