title: Add nullable foreign key in Rails
body: <p>Referencing to Rails 4.2 <a href="http://edgeguides.rubyonrails.org/4_2_release_notes.html#foreign-key-support" rel="noreferrer">add_foreign_key</a> support:</p>
  
  <pre><code>    # add a foreign key to `articles.author_id` referencing `authors.id`
      add_foreign_key :articles, :authors
  </code></pre>
  
  <p>How to create a nullable foreign key constraint, to allow the situation, where <code>articles.author_id</code> can be sometimes null?</p>