Add Associations section to Active Record basics guide [ci-skip]

The other Active Record guides are mentioned in the Basics guide as
well, so it makes sense to also shortly mention the associations and
where to find out more about them.

guides/source/active_record_basics.md

@@ -395,3 +395,23 @@ PostgreSQL, Oracle, and others.
 
 You can learn more about migrations in the [Active Record Migrations
 guide](active_record_migrations.html).
+
+Associations
+------------
+
+Active Record associations allow you to define relationships between models.
+Associations can be used to describe one-to-one, one-to-many, and many-to-many
+relationships. For example, a relationship like “Author has many Books” can be
+defined as follows:
+
+```ruby
+class Author < ApplicationRecord
+  has_many :books
+end
+```
+
+The Author class now has methods to add and remove books to an author, and much
+more.
+
+You can learn more about associations in the [Active Record Associations
+guide](associations_basics.html).