Update README to reflect growth of platform tests.
Update README to reflect growth of platform tests.

This README dates from when there wer eonly the single set of custom
acceptance tests. Update it now that there are more test suites present.

alphagov/paas-cf,jimconner/paas-cf,jimconner/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,jimconner/paas-cf,alphagov/paas-cf,alphagov/paas-cf,jimconner/paas-cf,alphagov/paas-cf,alphagov/paas-cf,jimconner/paas-cf,jimconner/paas-cf,jimconner/paas-cf