doc: add git mr command promotion

What does this MR do?

Add link git-mr which automates git merge requests fetching.

There are major and minor points to using the script:

• makes optional to specify remote name
• checks out only specific merge request. this is super important for repo like gitlav-ce
• sets up remote tracking, so the branch works like regular branch where you can push

I should be putting that benefits to the project own readme ;)