Add trait transient attributes to GETTING_STARTED

It might not otherwise be clear to readers that this is possible,
though it represents a worthwhile use case.

GETTING_STARTED.md

@@ -748,7 +748,7 @@ end
 create(:post).author
 ```
 
-Finally, traits can be used within other traits to mix in their attributes.
+Traits can be used within other traits to mix in their attributes.
 
 ```ruby
 FactoryGirl.define do
@@ -765,6 +765,24 @@ FactoryGirl.define do
 end
 ```
 
+Finally, traits can accept transient attributes.
+
+```ruby
+factory :invoice do
+  trait :with_amount do
+    transient do
+      amount 1
+    end
+
+    after(:create) do |invoice, evaluator|
+      create :line_item, invoice: invoice, amount: evaluator.amount
+    end
+  end
+end
+
+create :invoice, :with_amount, amount: 2
+```
+
 Callbacks
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