[ci skip] docs for add_references index option

- Add link for finding the addional options for index.
- Add example for unique index as this is a common requirement.
- Add link in guide for index options.

1 files changed, 2 insertions, 1 deletions

diff --git a/guides/source/active_record_migrations.md b/guides/source/active_record_migrations.md
index cd6b7fdd67..31f6f26b2a 100644
--- a/guides/source/active_record_migrations.md
+++ b/guides/source/active_record_migrations.md
@@ -246,7 +246,8 @@ class AddUserRefToProducts < ActiveRecord::Migration[5.0]
 end
 ```
 
-This migration will create a `user_id` column and appropriate index.
+This migration will create a `user_id` column and appropriate index.   
+For more `add_reference` options, visit the [API documentation](http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_reference).
 
 There is also a generator which will produce join tables if `JoinTable` is part of the name: