Adds Viable category maturity

Let's use this issue to discuss adding a fourth category maturity level, the naming of it and the definition. Straw man proposal added here.

I'll argue that there is a clear need for a fourth maturity level between minimal and complete and that by not having it we aren't clearly able to communicate our plans for maturity. The jump between minimal (a foundation to build upon) and complete (sufficient to displace) is too large and doesn't give us the opportunity to communicate to users and sales when we think a category is "ready for use" which is primary question they want answered.