Update readme with new integration test example using latest conventions

[StephenAWalsh]

A lot has changed since Nov 2016. Our testing codebase still relies on this helper and this new syntax works great.

[alexlafroscia]

Since this addon it still in use, although deprecated, this could go a step further an provide a test helper with the addon.

I created something like this for our application:

export default function setupRouteActions(hooks) {
  hooks.beforeEach(function() {
    this.owner.register(
      'helper:route-action',
      helper(([actionName]) => {
        const stubbedAction = this.routeActions[actionName];

        if (!stubbedAction) {
          throw new Error(`Missing route action stub for \`${actionName}\``);
        }
        return stubbedAction;
      })
    );

    this.routeActions = this.routeActions || {};
  });
}

ember-route-action-helper could include something like it in the addon-test-support tree so that everyone should share it.

[kexposito]

Yes please! we should update the readme with this