Include DatabaseCleaner usage when demoing FactoryGirl.lint

Because built factories create associated records, the database may not
be empty when the suite is run. This encourages developers to start
DatabaseCleaner and clean after linting is complete to ensure a clean
database.

Closes #619, #611, #620

GETTING_STARTED.md

@@ -88,11 +88,19 @@ RSpec.configure do |config|
   # additional factory_girl configuration
 
   config.before(:suite) do
-    FactoryGirl.lint
+    begin
+      DatabaseCleaner.start
+      FactoryGirl.lint
+    ensure
+      DatabaseCleaner.clean
+    end
   end
 end
 ```
 
+After calling `FactoryGirl.lint`, you'll likely want to clear out the
+database, as built factories will create associated records.
+
 Defining factories
 ------------------