Clarify documentation on using initialize_with

Using ignore blocks to avoid duplicate assignment is obsolete since
factory_girl 4.0.

Closes #622

GETTING_STARTED.md

@@ -873,20 +873,15 @@ end
 sequence(:email) { |n| "person#{n}@example.com" }
 
 factory :user do
-  ignore do
-    name "Jane Doe"
-  end
-
+  name "Jane Doe"
   email
+
   initialize_with { new(name) }
 end
 
 build(:user).name # Jane Doe
 ```
 
-Notice that I ignored the `name` attribute. If you don't want attributes
-reassigned after your object has been instantiated, you'll want to `ignore` them.
-
 Although factory_girl is written to work with ActiveRecord out of the box, it
 can also work with any Ruby class. For maximum compatibiltiy with ActiveRecord,
 the default initializer builds all instances by calling `new` on your build class
@@ -908,9 +903,7 @@ For example:
 
 ```ruby
 factory :user do
-  ignore do
-    name "John Doe"
-  end
+  name "John Doe"
 
   initialize_with { User.build_with_name(name) }
 end