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