Add a section on choosing work for the Scalability team

Why is this change being made?

We talked about this a bit in our weekly meeting, as well as in our Slack channel. This is my understanding of how we - ICs - pick something to work on.

@gitlab-org/scalability please take a look and see if it matches with your understanding!

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