Add changelog

CHANGELOG.md

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+# 2023-10-10
+
+- Update the instructions for setting a columns' non-null constraint.
+  Avoid using `modify/3` since that will include adjusting the type
+  unnecessarily, which will cause Postgres to rewrite the table.
+  Thanks @dhedlund for the report!
+
+# 2023-05-10
+
+- Add note about setting `migration_lock: false` on CheckRepoStatus when
+  the Repo is using advisory locks for migrations. Thanks @cgrothaus for the
+  report!
+
+# 2022-10-06
+
+- Ecto 3.9.0 supports advisory locks for migrations, which is a great way to
+  manage migrations when you need to create indexes concurrently. Add a note about
+  using that migration locking strategy.