Make implication of Global Optimization and Collaboration values explicit

In the days leading up to the feature freeze, as the pressure to finish one's assigned deliverables increases, engineers who are also reviewers or maintainers may find it hard to balance the competing and seemingly conflicting priorities of finishing up their own deliverables, vs reviewing their assigned MRs to help other people finish up theirs. In some instances this has led to reviewers' own deliverables being done ahead of the feature freeze, while deliverables of people who came to them for review missed the deadline.

One implication of our values of Collaboration ("Helping others is a priority, even when it is not immediately related to the goals that you are trying to achieve.") and Global Optimization ("Don't optimize for the goals of your team when it negatively impacts the goals of other teams, our users, and/or the company.") is that unblocking others is always a priority, even if it may be at the expense of one's own deliverables or other tasks, but I thought this could be made a bit more explicit.

@sytses I'd love to hear your thoughts!