Add loginCancelled to readme file

[david-byng]

The authService has only one method: #loginConfirmed().

      loginCancelled: function(data, reason) {
        httpBuffer.rejectAll(reason);
        $rootScope.$broadcast('event:auth-loginCancelled', data);
      }

Updating the readme to include info on how to use the loginCancelled method would make it easier for people to get started with angular-http-auth.

[moskeyombus]

Finding this issue saved me from pulling angular-http-auth out of my project entirely. PR opened to fix the documentation.