Add note about sequences and uniqueness [ci skip]

Closes #1393

GETTING_STARTED.md

@@ -52,6 +52,7 @@ Getting Started
   + [Without a block](#without-a-block)
   + [Aliases](#aliases-1)
   + [Rewinding](#rewinding)
+  + [Uniqueness](#uniqueness)
 * [Traits](#traits)
   + [Defining traits](#defining-traits)
   + [As implicit attributes](#as-implicit-attributes-1)
@@ -878,6 +879,22 @@ generate(:email) # "person1@example.com"
 
 This rewinds all registered sequences.
 
+### Uniqueness
+
+When working with uniqueness constraints, be careful not to pass in override values that will conflict with the generated sequence values.
+
+In this example the email will be the same for both users. If email must be unique, this code will error:
+
+```rb
+factory :user do
+  sequence(:email) { |n| "person#{n}@example.com" }
+end
+
+FactoryBot.create(:user, email: "person1@example.com")
+FactoryBot.create(:user)
+```
+
+
 Traits
 ------