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