text: Title: Preventing useless queries when listing entities
  Body: Hello,
  
  On an application with about 100k entries, listing them takes minutes. This surprised me because listing entries should be fairly quick, even if there are many of them. It appears that **for each entry** it produces a query to **each** relationship. This makes a huge number of queries. To understand if I did something wrong, I started from your own example in the documentation. I created 100 computers and 100 persons, related to a computer.
  
  Then I listed all the computers (with `/computers?page[size]=0`) and I asked SQLAlchemy to log every query. This confirmed that I had one `SELECT` on the `computer` table and as many `SELECT` on the `person` table  as there are owner of a computer. For instance, one of them:
  ```
  INFO:sqlalchemy.engine.base.Engine:SELECT person.id AS person_id, person.name AS person_name, person.email AS person_email, person.birth_date AS person_birth_date, person.password AS person_password 
  FROM person 
  WHERE person.id = ?
  INFO:sqlalchemy.engine.base.Engine:(19,)
  ```
  
  First: why is this query necessary? I mean the listing doesn't provide the detail of the person, so why retrieving this data? How could we prevent Flask-REST-JSONAPI from retrieving it?
  
  Second: if this query is necessary, why don't you have a join?
  
  Third: can I prevent this from happening to prevent huge efficiency losses?
  
  Thanks a lot!