Improve the MR guidelines from the CONTRIBUTING guide

What does this MR do?

This addresses a feedback from @jschatz1 (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9839#note_25100703) about specs that are considered backend code.

It also add details about approvals:

• you can require more than 1 approval (suggested by @DouweM in Tuesday team call agenda on @filipa's item)
• you can choose specific approvers if you want to (for very specific subjects, it can make sense)

@jschatz1 @DouweM Please review & approve.

added 75 commits

• 75f93979...3735266d - 74 commits from branch master
• cecf7328 - Improve the MR guidelines from the CONTRIBUTING guide

Compare with previous version

approved this merge request

@jschatz1 Could you please review, approve and merge? Thanks! :)

assigned to @jschatz1

Perfect @rymai thanks.

approved this merge request

merged

mentioned in commit 5f6c32e1