update README to combine axeOptions examples
update README to combine axeOptions examples

I changed the rule because it didn't make sense to disable color-contrast, and to provide an option for its check.  Also, I changed it to enabling a rule that is disabled by default, because I think it's better to show an example of enabling additional a11y checks rather than disabling.
trentmwillis/ember-a11y-testing,ember-a11y/ember-a11y-testing,trentmwillis/ember-a11y-testing,ember-a11y/ember-a11y-testing,ember-a11y/ember-a11y-testing