Updated association_basics.md [ci skip]

Updated association_basics.md to include how to write migrations for self joins

Update association_basics.md

Corrected the update

Update association_basics.md

Typo fix

Changed :employee to :manager

guides/source/association_basics.md

@@ -490,6 +490,19 @@ end
 
 With this setup, you can retrieve `@employee.subordinates` and `@employee.manager`.
 
+In your migrations/schema, you will add a references column to the model itself.
+
+```ruby
+class CreateEmployees < ActiveRecord::Migration
+  def change
+    create_table :employees do |t|
+      t.references :manager
+      t.timestamps
+    end
+  end
+end
+```
+
 Tips, Tricks, and Warnings
 --------------------------