Improve documentation [ci skip]

Add information about usage `uuid` type with `reference`

guides/source/active_record_postgresql.md

@@ -245,6 +245,7 @@ article.save!
 * [type definition](http://www.postgresql.org/docs/9.3/static/datatype-uuid.html)
 * [generator functions](http://www.postgresql.org/docs/9.3/static/uuid-ossp.html)
 
+NOTE: you need to enable the `uuid-ossp` extension to use uuid.
 
 ```ruby
 # db/migrate/20131220144913_create_revisions.rb
@@ -263,6 +264,30 @@ revision = Revision.first
 revision.identifier # => "a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11"
 ```
 
+You can use `uuid` type to define references in migrations
+
+```ruby
+# db/migrate/20150418012400_create_blog.rb
+def change
+  create_table :posts, id: :uuid
+end
+
+create_table :comments, id: :uuid do |t|
+  # t.belongs_to :post, type: :uuid
+  t.references :post, type: :uuid
+end
+
+# app/models/post.rb
+class Post < ActiveRecord::Base
+  has_many :comments
+end
+
+# app/models/comment.rb
+class Comment < ActiveRecord::Base
+  belongs_to :post
+end
+```
+
 ### Bit String Types
 
 * [type definition](http://www.postgresql.org/docs/9.3/static/datatype-bit.html)