update example

README.md

@@ -54,12 +54,28 @@ class NullPlan
   def pro?  = false
   def ent?  = false
 end
+```
 
+```ruby
 class Account
   belongs_to :plan, optional: true, null_object: NullPlan.instance
 end
 ```
 
+```ruby
+account = Account.create
+
+puts account.plan       # => #<NullPlan>
+puts account.plan.free? # => true
+puts account.plan.ent?  # => false
+
+account.update(plan: Plan.new(ent: true))
+
+puts account.plan       # => #<Plan id=1>
+puts account.plan.free? # => false
+puts account.plan.ent?  # => true
+```
+
 ## Supported Rubies
 
 **`null_association` supports Ruby 3.1 and above.** We encourage you to upgrade