Merge branch 'documentation/update_mr_contribution_section' into 'master'

Update Contribution Guidelines: Specify Merge Request To Master

There have been occasions where contributors submitted merge requests to stable branches thinking that was the way to get changes into specific releases. This results in wasted time and effort of the GitLab team responding to the merge request requesting that it be to master, as well as the developer who needs to redo the changes.

This merge request very simply updates the contribution guidelines to specify that MRs should only be to master, and that getting changes into stable branches will be determined by the GitLab Team.

cc @JobV

CONTRIBUTING.md

@@ -63,7 +63,7 @@ If you can, please submit a merge request with the fix or improvements including
 1. Add your changes to the [CHANGELOG](CHANGELOG)
 1. If you have multiple commits please combine them into one commit by [squashing them](http://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
 1. Push the commit to your fork
-1. Submit a merge request (MR)
+1. Submit a merge request (MR) to the master branch
 1. The MR title should describes the change you want to make
 1. The MR description should give a motive for your change and the method you used to achieve it
 1. If the MR changes the UI it should include before and after screenshots