Introduce a MR description template

.gitlab/merge_request_templates/default_mr.md

+<!-- title of the merge request -->
+
+# MR Title
+
+<!-- General merge request description.
+
+Write a small paragraph describing the scope of the merge request. Cite relevant issues, if any. Keep in mind that the text might be used to write a release report, so adopt a descriptive approach.
+
+-->
+
+## Change log
+
+<!-- List of relevant changes.
+
+Provide a list of the main changes introduced by this MR, possibly in the format of a bullet or numbered item list.
+
+-->