Update merge_requests.md with a simpler way to check out a merge request

The previous way was put under the
"By modifying `.git/config` for a given repository" section. The
disadvantage is that one has to modify `.git/config` for each repository
that one uses. The new way was put under the "By adding a git alias"
section. One has to create this git alias once, and then it will work
from any repository, without having to modify `.git/config` over and
over again.

doc/workflow/merge_requests.md

@@ -15,6 +15,21 @@ Please note that you need to have builds configured to enable this feature.
 
 ## Checkout merge requests locally
 
+### By adding a git alias
+
+Add the following alias to your `~/.gitconfig`:
+```
+[alias]
+    mr = !sh -c 'git fetch origin merge-requests/$1/head:mr-$1 && git checkout mr-$1' -
+```
+Now you can check out a particular merge request from any repository (it will use the `origin` remote) by:
+```
+$ git mr 5
+```
+This will fetch the merge request into a local `mr-5` branch and check it out.
+
+### By modifying `.git/config` for a given repository
+
 Locate the section for your GitLab remote in the `.git/config` file. It looks like this:
 
 ```