Add placeholder language for success criteria to wiki

In `wiki/What's-this-all-about….md`, I suggest adding some placeholder language for what constitutes success for the project, to answer questions like:
- How could success be evaluated or measured, both qualitatively and quantitatively?

> Both to get people to think about how they could tell if something was successful and worthwhile, and to consider whether they’d build metrics into the project, in code or in some other process.)
- Can success be measured in phases, perhaps along the lines of
- determining how much a need exists for different potential users and stakeholders
- determining how these people would judge something successful
- evaluating proposed solutions (prototypes, early versions, et al.) against success criteria
- evaluating different phased releases