Create the has_many relationship between Articles and Comments

 * The comment.rb file already defined the 'belongs_to' relationship.
 Now, we define the 'has_many' relationship in the Article model
 (article.rb). Each article may have many comments, but each comment is
 associated with only one article.
 * The object on the 'many' end (in this case a comment) should have a
 foreign key referencing the "one" object (in our case an article).
 * This foreign key is titled with the name of the 'one' object, then an
 underscore, then 'id'.
  - In the db/schema.rb file, you'll see a field named 'article_id' for
    the 'comments' table.