Add documentation for enum traits [ci skip]

This commit adds top-level documentation for the enum traits feature
adding in #1380

GETTING_STARTED.md

@@ -61,6 +61,7 @@ Getting Started
   + [With associations](#with-associations-1)
   + [Traits within traits](#traits-within-traits)
   + [With transient attributes](#with-transient-attributes)
+  + [Enum traits](#enum-traits)
 * [Callbacks](#callbacks)
   + [Default callbacks](#default-callbacks)
   + [Multiple callbacks](#multiple-callbacks)
@@ -1107,6 +1108,98 @@ end
 create :invoice, :with_amount, amount: 2
 ```
 
+### Enum traits
+
+Given an Active Record model with an enum attribute:
+
+```rb
+class Task < ActiveRecord::Base
+  enum status: {queued: 0, started: 1, finished: 2}
+end
+
+```
+
+factory\_bot will automatically define traits for each possible value of the
+enum:
+
+```rb
+FactoryBot.define do
+  factory :task
+end
+
+FactoryBot.build(:task, :queued)
+FactoryBot.build(:task, :started)
+FactoryBot.build(:task, :finished)
+```
+
+Writing the traits out manually would be cumbersome, and is not necessary:
+
+```rb
+FactoryBot.define do
+  factory :task do
+    trait :queued do
+      status { :queued }
+    end
+
+    trait :started do
+      status { :started }
+    end
+
+    trait :finished do
+      status { :finished }
+    end
+  end
+end
+```
+
+If automatically defining traits for enum attributes on every factory is not
+desired, it is possible to disable the feature by setting
+`FactoryBot.automatically_define_enum_traits = false`
+
+In that case, it is still possible to explicitly define traits for an enum
+attribute in a particular factory:
+
+```rb
+FactoryBot.automatically_define_enum_traits = false
+
+FactoryBot.define do
+  factory :task do
+    traits_for_enum(:status)
+  end
+end
+```
+
+It is also possible to use this feature for other enumerable values, not
+specifically tied to Active Record enum attributes.
+
+With an array:
+
+```rb
+class Task
+  attr_accessor :status
+end
+
+FactoryBot.define do
+  factory :task do
+    traits_for_enum(:status, ["queued", "started", "finished"])
+  end
+end
+```
+
+Or with a hash:
+
+```rb
+class Task
+  attr_accessor :status
+end
+
+FactoryBot.define do
+  factory :task do
+    traits_for_enum(:status, { queued: 0, started: 1, finished: 2 })
+  end
+end
+```
+
 Callbacks
 ---------