Add clarification for inexperienced users

We hit that issue ourselves and it took us weeks until someone pointed out our error.
As it feels like this could hit other users inexperienced with kubernetes too, I propose this note.

docs/migration.md

@@ -65,3 +65,17 @@ spec:
   old_postgres_configuration_secret: <resourcename>-old-postgres-configuration
   ...
 ```
+## Important Note
+If you intend to put all the above in one file, make sure to separate each block with three dashes like so:
+
+```yaml
+---
+# Secret key
+
+---
+# Database creds
+
+---
+# AWX Config
+```
+Failing to do so will lead to an inoperable setup.