Add section on When to scale

This came up in a conversation with @marcocitus and I'm opening an issue to capture it.

One of the questions we frequently hear from users is, "When is the right time to scale?" We've been internally using heuristics to inform users and we informally captured them in our multi-tenant database webcast: https://www.citusdata.com/blog/2016/11/07/video-and-slides-scaling-your-saas-database/

Slides 5-10 have a rough overview of heuristics we communicate to users. It would be helpful to add a section to our documentation that captures these heuristics.