Document how Growth team does visual reviews of MRs

Why is this change being made?

To document a recent conversation - gitlab-org/gitlab-design#943 (comment 286311450).

If this isn't the best place for this content, please feel free to move it elsewhere. I couldn't find the perfect spot for it.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what