Merge branch 'import-export-dev-doc-ee' into 'master'

Add Import/Export dev docs

See merge request gitlab-org/gitlab-ee!9131

doc/development/README.md

@@ -48,6 +48,7 @@ description: 'Learn how to contribute to GitLab.'
 - [Avoid modules with instance variables](module_with_instance_variables.md) if possible
 - [How to dump production data to staging](db_dump.md)
 - [Working with the GitHub importer](github_importer.md)
+- [Import/Export development documentation](import_export.md)
 - [Elasticsearch integration docs](elasticsearch.md)
 - [Working with Merge Request diffs](diffs.md)
 - [Permissions](permissions.md)