Add Merge Request template

Hi, I propose to use a MR template within the project in order to make sure that we do not skip information that might be relevant when asking for reviews.

As it's a template, it can easily be modified or removed when not needed (case in point: I'm not using it now). We'd need to have it in master, then activate the option in the repo.

See: https://git.xfel.eu/gitlab/help/user/project/description_templates

It's a template I've used in other projects, or here , which I think exhaustive enough.

@ahmedk @kamile @amunnich @tmichela

.gitlab/merge_request_templates/default.md

@@ -13,6 +13,9 @@
       Include details of your testing environment, tests ran to see how
       your change affects other areas of the code, etc. -->
 
+## Relevant Documents (optional)
+<!-- Include any relevant screenshot, elogs, reports, if appropriate. -->
+
 ## Types of changes
 <!--- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
 - [ ] Bug fix (non-breaking change which fixes an issue)