System specs: add note on transaction

Author: olleolleolle

features/system_specs/system_specs.feature

@@ -1,7 +1,7 @@
 Feature: System spec
 
     System specs are RSpec's wrapper around Rails' own
-    [system tests](http://guides.rubyonrails.org/testing.html#system-testing).
+    [system tests](https://guides.rubyonrails.org/testing.html#system-testing).
 
     System tests allow you to test user interactions with your application,
     running tests in either a real or a headless browser. System tests use
@@ -15,6 +15,8 @@ Feature: System spec
     uses the default `driven_by(:selenium)` from Rails. If you want to override
     this behaviour you can call `driven_by` manually in a test.
 
+    System specs run in a transaction. So unlike feature specs with
+    javascript, you do not need [DatabaseCleaner](https://github.com/DatabaseCleaner/database_cleaner).
 
     @system_test
     Scenario: System specs