Add clarity around linting and database_cleaner

Closes #645

GETTING_STARTED.md

@@ -99,7 +99,9 @@ end
 ```
 
 After calling `FactoryGirl.lint`, you'll likely want to clear out the
-database, as built factories will create associated records.
+database, as built factories will create associated records. The provided
+example above uses the database_cleaner gem to clear out the database; be sure
+to add the gem to your Gemfile under the appropriate groups.
 
 Defining factories
 ------------------