Docs on sideloading relationships

[myartsev]

If you guys agree that this belongs in the guides, I think it'd be helpful to have info on how to sideload relationships:

let blogPost = this.get('store').findRecord('blog-post', 1, { include: 'comments' });

https://guides.emberjs.com/v2.15.0/models/finding-records/

Perhaps it would also be interesting to update the data models used in the tutorial with a relationship that is sideloaded (e.g. a component that shows reviews for each rental). Just a little something to further illustrate the power of JSONAPI + Ember data.