add note about imports to template

packages/prisma-client/prisma/migrations/template.txt

@@ -1,5 +1,18 @@
 import { PrismaClient } from "./client";
 
+// NOTE ON IMPORTS:
+//
+//   You shouldn't import any code that can change as the project evolves!
+//
+//   It should be safe to import a library like `uuid` as its API is unlikely to change.
+//   For application code, it's better to copy it to the migration directory.
+//
+//   If you have to import something that may change,
+//   be prepared to keep the migration code up to date as well.
+//
+//   This is necessary because we want to be able to run old migrations at any point.
+//   For example, when we setting up a fresh database or making a reset.
+
 export default () => {
   const client = new PrismaClient();
   return client.$transaction(async (prisma) => {