docs(schemas): add migration file readme (#1984)

packages/schemas/src/migrations/README.md

@@ -1,3 +1,40 @@
 # Database Migrations
 
 The folder for all migration files.
+
+## Format
+
+The migration files are named in the format of `<version>.ts` where `version` is this npm package's version number.
+
+As for development, the `version` is "next" until the package is released.
+
+## Typing
+
+```ts
+type MigrationScript = {
+  up: (connection: DatabaseTransactionConnection) => Promise<void>;
+  down: (connection: DatabaseTransactionConnection) => Promise<void>;
+};
+```
+
+When the migration script is executed, the `up` function is called to alter the database schema.
+
+The `down` function is designed for the future downgrade feature.
+
+## Example
+
+```ts
+export const up = async (connection) => {
+  await connection.query(`
+    ALTER TABLE "user"
+    ADD COLUMN "email" VARCHAR(255) NOT NULL;
+  `);
+};
+
+export const down = async (connection) => {
+  await connection.query(`
+    ALTER TABLE "user"
+    DROP COLUMN "email";
+  `);
+};
+```