DOC: Gitlab merge template.

.gitlab/merge_request_templates/General.md

+Insert a meaningful description for this merge request here.  What is the
+new/changed behavior?  Which bug was fixed?
+
+---
+
+Add the text above this line to the git comment.
+
+# Check list for the author #
+
+- [ ] All tests pass. (style, unit/integration, static code analysis)
+- [ ] What is the main focus for the reviewer?  What is the core intent of this
+      MR?
+- [ ] How to set up a test environment.
+- [ ] Annotations in code (Gitlab comments)
+  - Intent of new code
+  - Problems with old code
+  - Why this implementation?
+
+# Check list for the reviewer #
+
+- [ ] Tests pass
+- [ ] Do I understand the intent of this MR?
+- [ ] Does the test environment setup work?
+- [ ] Is the intended behavior reproducible in the test environment?
+- [ ] Code overview: Which functions/classes are relevant for understanding the MR?
+- [ ] Work through the open comment threads.
+- [ ] Are the doc strings / comments up-to-date?  Or have there been behavioral
+  changes without amending the documentation?
+- [ ] Are there spezifications? Are they satisfied?