Updated README with new evaluation criterion.

README now has new "Interface Segregation" criterion, describing requirement for specifying service-object input by interface rather than class name.

RSpec: 4 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 9 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2