add merge request template

.gitlab/merge_request_templates/DEFAULT.md

+Thank you for making a Merge Request. Before you merge this, please make sure
+that you:
+
+- [ ] Manually tested chart changes with the openappstack bootstrap script
+- [ ] Created an MR in openappstack/openappstack that applies the changes from this MR
+- [ ] Tests in that MR in openappstack/openappstack succeed
+- [ ] Updated chart version number