Add merge request default template

@timok, @heck, @bernd, following our thoughts on a seminar on what is important in merge requests, I thought it could be helpful to have a descriptive default merge request template appearing always when someone opens a merge request.

I basically copied the text from our merge_request.md template but added a check list what should be considered before asking for a review. This should be extended in case we want to do this. I pushed this to ask for you opinion on this...

TODO:

• if we want to keep this layout, we should probably delete the other merge request template file?