Currently, we have very well documented reel.Handler instances.  In
fact, documentation could be guaranteed via API in the near future.

However, reel.Handlers are really just the building blocks for
composing larger test cases.  As such, we need to be more pedantic
about documenting our Ginkgo test specs.

Since Ginkgo does not appear to utilize or enforce any interface, this
is currently nearly impossible to do programmatically.  This change is
a low cost addition to README.md which explains in plain english what
each Ginkgo test spec actually tests.  It also provides hints for
well-aligned future work, but makes no commitment to delivering such
tests.

Signed-off-by: Ryan Goulding <rgouldin@redhat.com>