MAINT: add merge request template

.gitlab/merge_request_templates/Default.md

+# Summary
+
+*Insert a meaningful description for this merge request here:  What is the new/changed behavior?
+Which bug has been fixed? Are there related issues?*
+
+
+# Focus
+
+*Point the reviewer to the core of the code change. Where should they start reading? What should
+they focus on (e.g. security, performance, maintainability, user-friendliness, compliance with the
+specs, finding more corner cases, concrete questions)?*
+
+# Testing
+
+*How to set up a test environment for manual testing? Is there any manual testing needed?*
+
+# Check List for the Author
+
+- [ ] All automated tests pass
+- [ ] Reference related issues
+- [ ] Appropriate user and developer documentation for new code
+  - How do I use the software?  Assume "stupid" users.
+  - How do I develop or debug the software?  Assume novice developers.
+- [ ] Annotations in code (Gitlab comments)
+  - Intent of new code
+  - Problems with old code
+  - Why this implementation?
+
+
+# Check List for the Reviewer
+
+- [ ] I understand the intent of this MR
+- [ ] All automated tests pass
+- [ ] Appropriate user and developer documentation for new code
+- [ ] The test environment setup works and the intended behavior is reproducible in the test
+  environment
+
+For further good practices have a look at [our review guidelines](https://gitlab.com/caosdb/caosdb/-/blob/dev/REVIEW_GUIDELINES.md).
+
+/assign me
+/target_branch main