Docs for other testing packages

[jackfirth]

RackUnit doesn't handle everything, nor should it. It would be useful to have a few words in the docs about community packages related to RackUnit, such as:

• Packages that extend or integrate with RackUnit to make certain tests easier to write
• Packages that provide alternate testing frameworks and what they do differently from RackUnit

Maybe this should include more words about how rackunit/log is used as the "standard" interface for getting raco test to count tests and how that applies to alternate test frameworks?

Arose during discussion of #83