Add `validates_presence_of` example with `on` option

README.md

@@ -34,6 +34,7 @@ describe Post do
   it { should validate_uniqueness_of(:title).scoped_to(:user_id, :category_id) }
   it { should validate_presence_of(:body).with_message(/wtf/) }
   it { should validate_presence_of(:title) }
+  it { should validate_presence_of(:title).on(:update) }
   it { should validate_numericality_of(:user_id) }
   it { should ensure_inclusion_of(:status).in_array(['draft', 'public']) }
 end