DOC: Add a gitlab template for request for changes

.gitlab/merge_request_templates/request_for_changes.md

+## Title of the Request for Changes
+
+### Summary
+
+Gives a short summary of the changes.
+
+### Rationale
+
+Explain the rationale for the changes (possible link to a Request For Comments or to an issue).
+
+### Implementation Details
+
+#### Classes and files
+
+Give an overview of the implementation: main changes made to classes, files and module. Do not paste complete diff, as it is available in the merge request already.
+
+#### Applications
+
+Describe any changes made to existing applications, or new application that have been added.
+
+#### Tests
+
+Describe the testing strategy for new features.
+
+### Documentation
+
+List or link documentation modification that were made (doxygen, example, software guide, application documentation, cookbook).
+
+### Additional notes
+
+List remaining open issues if any, and additional notes.