Add information about moving migrations

Add to docs paragraph about a way of moving EE migrations to CE
if needed

doc/development/migration_style_guide.md

@@ -423,3 +423,9 @@ _namespaces_ that have a `project_id`.
 The `path` column for these rows will be renamed to their previous value followed
 by an integer. For example: `users` would turn into `users0`
+
+### Moving migrations from EE to CE
+
+When migrations need to be moved from GitLab Enterprise Edition to GitLab Community Edition,
+a migration file should be moved from `ee/db/{post_,}migrate` directory in the `gitlab-ee` project to `db/{post_,}migrate` directory in the `gitlab-ce` project. This way
+the schema number remains intact, there is no need to modify old migrations, and proper columns, tables or data are added in the Community Edition.