Add configuration_spec

Now that spec coverage is no longer embarassingly minimal, update the
readme section to reflect it.

Change-Id: I4b45b90f392f6192e50cbfd0a6cc7b1262d4f0e9
Reviewed-on: https://gerrit.causes.com/23256
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>