Add FactoryGirl Sequences as a ruby til.

ruby/factory-girl-sequences.md

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+# FactoryGirl Sequences
+
+[FactoryGirl sequences](https://github.com/thoughtbot/factory_girl/blob/master/GETTING_STARTED.md#sequences)
+are often used inline for unique values such as emails:
+
+```ruby
+factory :user do
+  sequence(:email) { |n| "person#{n}@example.com" }
+end
+```
+
+However, a sequence can be defined on its own
+
+```ruby
+FactoryGirl.define do
+  sequence :email do |n|
+    "person#{n}@example.com"
+  end
+end
+```
+
+That means it can be invoked outside the context of a factory
+
+```ruby
+> FactoryGirl.generate :email
+=> "person1@example.com"
+> FactoryGirl.generate :email
+=> "person2@example.com"
+```
+
+Or it can be used as a shared sequence across multiple factories
+
+```ruby
+factory :customer do
+  ...
+  email
+end
+
+factory :admin do
+  ...
+  email
+end
+```